Destiny 2

Update (10 Nov 2020): Requirement has been changed to completing a Nightfall strike on Master difficulty. On this difficulty, matchmaking is disabled and you would likely have to LFG. Leviathan has been vaulted as of 10th November 2020.

Edit: Thanks to I Achi I, we have confirmation that this does not stack when completing the Grandmaster difficulty. As such, you would have to do 2 separate runs to unlock this and the following:
This is not really a guide, more of a confirmation that you CAN join in at the final stage (Calus) and beat that to get the achievement. I didn't manage to complete any of the initial stages, but managed to defeat Calus with a fireteam by joining in.

As per usual with Destiny, the achievement may not pop immediately after you finish the activity. In-game, go to Settings>Log Out>Switch Profile. You'll go back to the start menu, pick a Gamertag, pick your character, and your achievement will pop once you load into orbit.

Just a simple video guide for each of the 4 trials plus the main hub.

Step #1: Main Hub(Castellum)

Step #2: Royal Pools

Step #3: Pleasure Gardens

Step #4: The Gauntlet

Step #5: Callus Boss Fight + Achievement!

To add to this I've got some tips for anyone looking to go through with a 5-man team rather than a full raid team - This pre-supposes that you have some knowledge of the mechanics of each raid section but does cover the basics for those looking for a reminder or assistance generally

Castellum:
Opening the doors is possible with a 5 or even 4 man team. The entrance to the pleasure gardens is certainly the hardest here as it has the most ways for enemies to flank in and the steps and pillars immediately in front of the plate do reduce line of sight.
Two (or more?) warlocks with rifts helps (particularly if it gives mix of buffs). Ward of Dawn can also be used to add weapons of light behind the standards but damage dealing supers are more useful once the heralds emerge.

Be very clear about what you call the approaches. Most of the time if someone goes down, they'll come back to the fight faster if you revive them even if the counter has ticked over but if you are defending the entrance to the baths, and a lesser extent with the gauntlet, the respawn point is just at the side so they can come straight back in (possibly behind a councillor ..)

If you don't have someone who can solo the standard bearer, then it's best to learn the underbelly ...

I find Skyburners Oath useful here as it can pass through the Phalanx shields. It's also surprisingly easy to run low on kinetic ammo here so weapons you can aim well are key.

Gauntlet:
Start with a person at each of the signs as normal with the fifth person assisting whomever on the ads.

Once the runners enter, you need to deliberately enter a delay so that you can have one person following each of the runners and the last has time to run through the centre to get line of sight onto a chevron for the double hit needed. This can be by having one of the runners not pick up their first orb, or by each runner having to wait a few seconds before someone stands on the pillar to show which row to shoot. There is enough time to run half way around the track before the charge detonates. It is also possible to trigger the last section with only 5 charges returned to the centre.

On the group run, you still need to decide who misses an orb each time but there is a little more flexibility. As long as everyone clears the second gate having collected at least one orb, there is time to get the minimum three orbs back to the centre.

The loot chest appears near the platform of cups - if you get on the elevator before collecting the loot you can miss it!

Pleasure Gardens:
Two collect the crystals as normal to start but then one drops their crystal and collects the remaining pollen spore so that four collect the charge and only one remains guiding from above. If you can get 55x or better on the buff, it should be relatively easier to deal enough damage to a hound to kill it (prioritize the two nearest the safe room) - as a titan, one pulse grenade and a clip from a 335 Merciless will do it for instance (which from M'orn still leaves enough time to get back to the safe room). Each person heads for one dog for the damage phase.
As dogs are killed, the time available to collect the buff decreases but you are able to move faster in the area's that the dog is killed in (hence why the priority).

On the second round, if your dog is still up, deal damage there as normal and anyone who has taken out their dog head for the dog that has had no damage yet - for us this was the dog most opposite from the save room (Va'ase). As long you were able to get some buff, you should be able to clear the dogs within 2 rounds.
We also checked (somewhat by accident) that a 5 man team with a x12 buff can reduce an entire dog in about half the time available (presuming supers, grenades and heavy ...)

With 6 dogs, other than the two nearest the save room, it's best to get into the trees or the rocks then go for the other spore locations from there. There is a sneaky little single person cut through the rocks just south of the R2 spore as well which helps when coming across from rocks

Once you know the layout, this is probably the easiest to complete with 5 persons.

Loot chest will appear on top of the safe room to one side

Royal Pools:
For this, you immediately go into a round of swopping. We choose to always move to the next clockwise platform but choose what works for you, remembering that you need to be cycling around the whole map. You will likely have move twice (so if I start at Cups, I will certainly get to Hounds). The protection buff will likely start to run out (it will certainly run low) by the end of the first full cycle - as long as someone is coming to replace you, a few seconds doesn't make a huge amount of difference (if using the example above, we haven't locked them in place whilst I'm on Hounds something has gone wrong).

For the damage phase, having a ward of dawn off to the side for weapons of light is useful (don't put it in the centre!). Supers are useful for dealing damage to adds (especially if they can also serve dual purposes (so solar supers are useful) to clear enemies and also be used to attack the crystals.

it's certainly doable in two rounds - just keep an eye on the buff when in the middle and make sure that someone is checking behind for additional bathers when attacking crystals. remember as well that there are councillors to both melee and shoot in the middle before running back.

Loot chest will appear in the middle room

Throne Room:
hah!
However, in all seriousness this should be doable - it'll likely take 3 damage phases though. At the beginning, hang back and clear as many as the ads as possible. You'll need everyone to have a high dps set-up. A 450rpm auto rifle in the kinetic slot, trace rifle in energy (coldheart or prometheus lens) and a cluster bomb rocket launcher is one good load out. If you don't have such a heavy weapon, then Merciless and an accurate energy rifle also works. However, most important is to use a weapon that you are consistent in landing shots on target - if using an auto rifle/SMG means that some bullets are likely to miss targets then switch to whatever weapon works for you.
No swords on this stage though or in your face shotguns

When the first councillors appear, leave them alone
When everyone is 'ported to the void space, send two back. If possible have a titan with rally barricade and a warlock with enpowering rift remain in the void. The three call the symbols in turn and deal with the projections and psions. The psions are likely to use their ground attack so prioritise killing them - you may need to switch targets to get the angles on a different psion if it consistently launches a guardian above the space. Avoid the ramps, there will be four different sets of symbols to call out then a fifth wave of psions.

At this point, everyone meets in the middle. The titan puts their rally barricade down and the warlock puts their rift up. The face begins spewing out skulls which need to be shot. This is when the trace rifles work great. The arc souls will also target the skulls and aim to shoot as many as possible. The skulls do cause damage if they hit you.

After a little time orbs will spawn at the front (several seconds after the class abilities wear off) Grab an orb to return to the throne room

The two in the throne room just need to stay alive whilst this is happening, ideally one to each side of the room. If the councillors have been left alone, there should be no ads in the room to start with though they will start to appear. As the players in the void call symbols, listen out for the symbol that is not mentioned and kill that councillor. This will happen 4 times.
Then Calus will begin to cast a new attack. He'll have a shield which needs to be destroyed to prevent a party wipe.
You should be aiming to nearly remove it then wait as long as you dare (you will be taking damage and the screen going white). The longer you leave it, the more skulls the void team can destroy and the higher the damage buff is for the damage phase - just don't leave it too long!

Calus also will throw something that looks like a solar grenade - if you get hit by these, then someone in the void space will be flung up and forward. If the appropriate councillor has been killed then the next barrier can save them but try to avoid them where possible.

When everyone comes back to the throne room, pick one of the platforms for everyone to jump to - it's easier to start on one of the two nearest the entry door. Again placing a rally barricade and empowering rift helps here. When on a platform, the damage buff takes effect and you have a few seconds to unload on him. When he rises his arm, he will hit the platform with an insta-kill attack so everyone needs to move and head to the next platform on the circuit (whichever the group choses). If anyone goes to a different platform, the damage buff will be used up on his next attack so it helps to coordinate which way around the group moves.

You need to be doing enough damage to remove over one section of his health by the time you go around four platforms. Use heavy weapons, grenades, supers.

A note on supers - solar are best for all three classes. Nightstalkers tether if it gives a damage buff to teammates can be useful, and arc supers for those in the throne room if the ads are providing difficult but otherwise void and arc don't seem to be the best choice.

Once you'd done a round, then it all starts again. Major change this time in the void is that a number of holes will have appeared in the ground in the void. Try to avoid these as well but if you jump as you fall in the hole, it's possible to mantle out. If you try to jump over them, you'll be sucked forward off the play space.

On the next damage phase, remember that Calus will start to use the gun (treat as if him raising his arm). When his chest glows red, that means his critical spot has moved from his head. When he goes down to the final third, he may not have the stagger animation but a ball of fire will engulf the platform the team is upon - you still need to move away to avoid the insta-kill.

Third phase is the same again but harder.

If you get all of his health down, he will start to cast the light shield attack again. At this point, just hit him from where ever you are in the room (keep moving as the dogs will still be attacking) - party wipe or win!

The chest will appear down the hole that appears after Calus is defeated - check out what lies beneath ...

Underbelly:
This can be opened by a single person by using the switches under the entrance pathway then heading up past the launchers and across the river. You will gain access to either the starting section for the week, or to any completed sections so it's possible to bypass the Castellum (for instance if you are still waiting for people to join the team, or someone is struggling with that fight) - and you'll need to access this section in any case to use the keys you get from each completed raid section.

The watchers will lock down at least one chest location if they have chance to spawn adds (engine room I think) so bringing a team down helps for that but just for gaining access to different sections coming in solo is fine.

Some chests (such as ventilator) are accessible for solo players but any room with watchers in will need the player to either avoid the red gaze and reach the chest (the watchers won't notice anything outside of the red gaze so you can pass right by them) or for four persons to each pick a watcher and eliminate it before the shield comes up (around 4 seconds).

Do be aware that you can reset a room by clearing ads, then watchers (watchers bodies explode after a few seconds after being defeated) then going back out through a door until it closes and re throwing the switch. However, watch out in places near to each other (Transfer and Armoury for instance) as activating watchers in one can seal the treasure doors in both ...

If you are just looking for chest locations, then any door that looks more ornate than the rest probably has a chest - I think there are sometime like 11 chests and I'd recommend grabbing one of the various maps from a web search

Hope this helps