Gems of War

Uber Doomskulls are a new type (first actually) of Heroic Gem, now part of Gems of War as of update 5.5 .
When matched or exploded, they deal +10 damage to the first enemy and explode a large area around themselves (much like a more powerful Doomskull, hence the name).
**Edit** Please note that as of the beginning of Campaign #4, 'Moon Flask', Lycanthropy gems have replaced Uber Doomskulls as the active Heroic gem. This means that Uber Doomskulls are currently NOT AVAILABLE IN GAME until they return, as there are NO troops that can summon nor convert gems to Uber Doomskulls in game. It is very likely that Uber Doomskulls will make a return in the future, probably during the break week in between this campaign and the next. Otherwise we'll have to wait for the devs to 'activate' them again, and until either thing happens, this achievement is UNOBTAINABLE.

My method for unlocking this achievement was this:

Explore 12 in Khaziel (kingdom does not matter for Uber Doomskull drops, I picked it for hearty troops and skull reduction traits, especially Stonehammer in the mythic fight).
Highest explore you can do is best, as the enemy troops will have the most armor/Life and the battle will last longer.

The team I used:
Greed (Gold elite)
Bronzelock Pistol with Tidecaller class (Mana Source at class lvl20 for 50% mana start, Water Mastery at lvl70 for extra blue mana from blue matches, and 3rd class trait for exploding blue gems on 4+ matches)
Zuul'Goth (lvl20, but did not use at all for this farm)
Cedric Sparklesack (lvl20, also hardly used during this farm)

Please note that your first two troops here are going to be doing all the work. Your back two troops can be nearly anything you think will help you if things start to go sideways while you are farming.
Newer players may need to do lower level explores as your stats will likely not allow you to do E12 this way. Test different levels to find the one that works for you.

Method:
Start an Explore 12 run in Khaziel.
Every fight, all I did was cast Greed immediately, then match brown or blue (if even needed, sometimes starting explosions filled hero weapon right away) to fill Bronzelock Pistol.
Cast weapon on first troop, this will destroy (not explode) most of the board (or all of it, if you are like me and an end-game player with high stats).
The destroyed board will fill Greed up, and most likely also refill your Hero weapon. Rinse and repeat, continuing to cast Greed every time it is full so he can tank skull hits, and keep firing weapon at first troop, until they're all dead.
Do not match skulls, except on first or second turns of the match while your Greed is still a bit weak; there's no need to lose a turn doing so, this team loops often (though not always). Don't cast Zuul unless for some reason you end up with Greed gone and your hero in danger. If Greed does get killed, use Cedric to summon a Bombbot as a replacement tank (This is less ideal as bombot will briefly manablock Hero.)
The goal here is to 'refresh' the board by destroying gems constantly, giving you more chances that the Uber Doomskulls will drop. You *can* match them when they appear (if they are matchable) but I actually never bothered with this; I just kept firing my weapon and destroying the board, which got rid of the Uber Doomskulls and counted towards the achievement (checked after each match).
Doing this, I unlocked the achievement after just over 3 Explore 12 runs (this includes the mythic boss fights each time, so about 20 fights total).

Notes:
Zuul'Goth is on my team but I literally never used him, as the goal was not to end the matches early. This team is my reliable gold-farming E12 team, I made no changes to this existing team. If you don't have Zuul yet, you can probably stick just about anything there instead; though if what you put there mana blocks Cedric, you might want to move Cedric up one slot. Cedric is there to summon Bombbots to replace Greed if he happens to get killed (only happened in one match of this entire achievement run, due to a Bonestorm on start).
I seemed to average between 2 and 3 Uber Doomskulls per battle with this method, though I had one or two matches where none spawned, and one match where 6 spawned (and frustratingly, the enemy AI matched 3 of them against me!)

This achievement is now possible with the help of the Doomed Gargoyle troop and its Doom Stone charged attack.

Convert a chosen Mana Gem to an Uber Doomskull.

It uses 12 brown/red mana and drops an Uber Doomskull at a place of your choosing - pick a gem next to two existing skulls to make a match.

I pulled mine from a glory chest, but it is also part of the Nexus trial team.

As of October 24th, 2021, the Soul Harvest Event is taking place for 7 days until the 31st and Uber Doomskulls have been spotted on several gametypes. I have linked my video with notes in the bio covering the info, but I'll sum it up here! There isn't any clarification if the Uber Doomskulls and the Lycanthropy gems will continue past this event so it's safe to assume that it ends on the 31st. It took me a total of 7.5hrs starting at 32% so most likely will take 10-12hrs if starting at 0% (This is also based on luck and frequency).


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