After engages a player

By batat, in Rules questions & answers

Troll bert has forced actions: if

Forced: After Bert engages a player, sack 1.
Forced: Return Bert to the staging area at the end of the combat phase. The engaged player may raise his threat by 1 to cancel this effect I

If I decide to pay 1 thread, do I need to sack another hero/ally in next round?

When After engages occur?

No, he is already engaged when you start the next round, so he does not sacK during the engagement phase.