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As Cthulhu, reach full stacks of Sanity Break in a single match.

Sanity Break is Cthulhu's 1st ability. It applies stacks of Torment, along with the rest of his kit. On reaching 4 stacks of Torment, enemies are afflicted by Insanity. You can tell when an enemy god is insane as they will have a red/purple coloured aura around them (you passive meter will also show one eye open for each insane god near you). Once they have been driven insane, if you hit them with Sanity Break, their insanity will be consumed and you will gain a permanent stack on the damage mitigation that Sanity Break provides. It stacks 20 times.

So, send a god insane then consume their insanity using your 1, then repeat this 20 times.

This will likely be an awkward achievement.

You can stack Torment in a variety of ways. The last hit of your basic attack chain applies 1 stack if it hits an enemy god, your 1st ability applies 1 stack if you hit an enemy god or 2 stacks if you hit them while they are facing you, both hits of your 2nd ability apply a stack each (so hitting both will apply 2 stacks), hitting an enemy god with any part of your 3rd ability (his dash) also applies 1 stack. Finally his ultimate applies a stack of torment in a decent sized area around him. It stacks every second or two, and increases in pace if the enemy gods are facing you.

The best way to earn this is in Arena. Enemies bunch up a lot and are far more likely to be hit by your abilities. I'd recommend saving Sanity Break as the final ability to use so you haven't sent someone insane only for them to escape before you can consume it. You can build a mix of protections and cooldown reduction to allow you to get in near people and actually apply Torment and the cooldown will help with how often you can apply it.

If you struggle in Arena, try Conquest or Joust. As the game is much longer, you'll be able to focus on one or two people in a lane and build up stacks with having everything dumped on you.

While working on this, you can try sending each god on the enemy team insane, which would net you Cthulhu's other achievement as well.

The other guides here explain how to get the stacks to do this so reference those for that part.

As for how to get this:

Arena is the way to go. Its easy to keep track of how your pacing is going as far as getting the stacks and for the most part surrenders are rare. Use that knowledge to your advantage.

This guide is for playing with at least 2 other folks but could be done with random players with some luck. You need one friend to play as Ares and one to play Kherpi. Both of these provide CC against enemy players and for the most part lack damage. The hardest part of this is not killing players too quickly or having them run away. Have your 2 friends chain their CC together, call out who you are going for and build the stacks on that God. Rinse and repeat on your way to 30%. I had this done with about 150 tickets left for both teams. If you can find 2 other folks to play with you, have them add in more gods to help hold enemies in place (Ymir, Baron, Etc). Two other friends is all that is needed though.

As for a build, Cooldown along with defenses is your friend. Get to 40% as quick as possible. Build the items with cooldown that have protections against the enemies that are dealing the most damage to you.

This achievement can be annoying trying it solo. So round up two folks and grab this in arena with little issue.

Happy hunting.

As Cthulhu, reach full stacks of Sanity Break in a single match.

You gotta get consume 20 stacks of insanity with (1) Sanity Break.

To get one stack of insanity you need to apply 4 stacks of torment to an enemy god

All of Cthulhu ability causes 1-2 stacks of torment
Starting with (1) Sanity Break (which is also the ability to consume) your enemy get 1 stack for hitting them with the ability and 2 if the enemy god is looking at you.

With (2) The Mire ability is AOE that strikes twice each strike is 1 stack of torment.

With (3) Rushing Terror is dash thats provides one stack with a hit and 2 stack if the tornado from behind hits as well( you can't hit at the same time)

With (4) Descend into Madness it provides stacks if the enemy in is in in radius of 50 (I don't know how long they gotta be near you but its like 3-5 seconds) if they are looking at you it progress faster.

With Cthulhu 3rd basic chain strike he provides 1 stack of torment (only on the 3rd strike)

I usually would start the fight with (1) "Sanity Break" than I use (2) "The Mire" and than end it with (3) "Rushing Terror" and by that time (1) "Sanity Break" is back up
you know someone has insanity because the Eyes where your passive is will be open 5 eyes 1 for each enemy god if an eye is open that means a enemy god who's close to you is infected hit them with your (1) sanity break to consume it once the passive hits 30% you did it

I found that clash was the easiest mode to achieve this is on as enemy are close together so you can infect multiple at once and you have more time than you would in arena to secure this achievement. Focus on other tanks at they are more willing to stay in lane the whole time cause as soon as you infect them you got 20 seconds so secure it or hit them with another ability to prolong it.

The items i went with are Shogun's Kusari, Hastened Ring, Chronos' Pendant, Pythagorem's Piece, Specialist's Blessing and Mantle of Discord

Shoguns and hastened for att speed as his 3rd basic hit counts as a stack
Specialist's Blessing, Chronos and mantle for cool down
Pythagorem for sustain
Specialist's Blessing and hastened for movement speed
I started with Chalice of Healing,Specialist's Blessing, tier 1 boots (going in to Reinforced Shoes)

Remember you are not trying to kill anyone just stacking your passive. you gotta try to go for this the whole time you will not get it for casually playing.( i got it at the 22min mark)