I haven’t seen the +10 doomskull in quite a bit. They showed up almost unannounced, and seem to have disappeared just as unceremoniously. They still list it as a possible skull though, on the skull count table.
I think that was the lycan gem for that campaign. From what I understand it will be a rotation? We wont see the purple ones drop in going forward for example. Those I believe can only be made by the troops released.
You can still get them from a random storm.