The Wretched

The ability grants you control of all creatures that are blocking it as the ability resolves. This will include any creatures that were put onto the battlefield blocking it.

Any blocking creatures that regenerated during combat will have been removed from combat. Since such creatures are no longer in combat, they cannot be blocking The Wretched, which means you won’t be able to gain control of them.

If The Wretched itself regenerated during combat, then it will have been removed from combat. Since it is no longer in combat, there cannot be any creatures blocking it, which means you won’t be able to gain control of any creatures.