Ok looked it up: Hitting 100 isn't necessarily instant death, I think, but if they don't start to quell to reduce the number, there's a couple effects that can happen: they explode, attract a mass daemon possession event upon your army, or a variable sized circle of warp storm around it

For those of us not in the know, does it burn itself out?

I dont know the system, but I know thats fucking bad

I once gave 40k, the tabletop a try. Their smaller skirmish version, killteam, I think. I had like 5 guys and turn one, one of them exploded himself, damaging the rest.

It's possible I misunderstood the rules, but the way we played was a better story. I lost, of course.