I Am One With The Force
The Force Is With Me
Always Leads: Infinity Gems
Thanos gets -2 Attack for each Infinity Gem Artifact card controlled by any player.
Master Strike: Each player reveals their hand and puts one of their non-grey Heroes next to Thanos in a "Bound Souls" pile.
They stay next to Thanos. If it said "Captured" then we could argue that they would be rescued like Bystanders however this is not the case. Thanos essentially KO's one of your cards and it goes into a special deck next to him.
Further evidence that this is correct is one of his tactics:
Keeper of Souls wrote:
Fight: Gain a Hero from Thanos' Bound Souls pile. Then each other player puts a non-grey Hero from their discard pile into Thanos' Bound Souls pile.
If you gained Heroes after beating him each time, this Tactic would have no effect as you would gain those Heroes, then you would gaina hero from a pile that would be empty, then all other players put a Hero in that pile.
Thanos is one of the tougher Masterminds for good reason