Story related, can’t be missed. Beating the final boss of the game can be a tricky fight if you don’t know what you’re doing. I’d recommend a horde of 20 browns, 10 reds, 10 greens and 5 blues (10 if using the +5 horde size helm). Put them on a guard marker and simply run around the Devourer at during the first part of the right, your reds and greens will hit the pustules while your browns will take care of any zombies than come at you, the blues keeping your minions standing though you shouldn't have too many die on you. You'll also need to sweep the blues around every so often if too much blue goop starts littering the area.
In the second section you have to use the raised platforms to hit the higher pustules, send your 10 reds up there on a guard marker and use the rest of the horde to defend against the zombies. The final section of the fight is the same as the first, except when you destroy the 2/3 pustules that regenerate each turn your then need to destroy the one on the Devourer’s head to make Solarius appear, you can then drain him while again your minions take care of any enemies that appear, these will generally be stronger that the first section with Unicorns and mutated minions attacking, you may want to get your overlord stuck in (especially on the Unicorns). Simply rinse and repeat.
The key to this fight is your control of minions on the marker. It’s just like sweeping them, use to select the marker and to move them (you can move the Overlord as well with at the same time) only you have more control and when in formation they defend much better. Easy enough to finish the fight with your full 45/50 minions if careful.