Not all at once, have them start coming back one at a time as time goes on

(A) with items and powers they gained in the meantime wherever they got sent to
(B) and team up to fuck up the party, in squads of increasing power level since the party has been meeting increasingly powerful enemies of course
(C) the same, but changed a little bit in unsettling subtle ways that get less and less subtle and more and more wild and empowering as the story continues, like Pet Semetary

Pick one. Start the first little challenge they run into right after they disintegrate some massively powerful BBEG, so the realization of what they are in for now and the anticipation of what horror is in store at the end of the arc can build and build and build

Caution, they may say “fuck it” and still keep disintegrating stuff, or experiment with disintegrating each other once they know there are strengthening effects, things like that, so be prepared for that