I was playing MSH limited and cast Super Hero Civil War, targeting only one creature, opponent's 5-drop 3/6 that taps for mana and gives additional counters. I passed the turn with a Spider-Man on my side of the field, I think opponent had the rare blue Loki 2/1 ok their side. During their turn they cast Pet Avengers and used it's power up ability, then passed back to me. Then, when my saga went to progress to the next chapter it had 3 counters on it, and prompted me to choose a creature to fight another one. After choosing to have the 3/6 fight the Loki, the ability from chapter 2 appeared ok the stack, giving my creatures +1/+1 and vigilance. Both abilities then resolved, and the saga sacrificed itself. Haven't ever encountered anything like this before. I was playing on mobile, on Android and on 5G. I unfortunately didn't get any screen shots; I think I was a bit too confused and dumb-founded at the moment.