The Devil Treasure Room generated will be the same as the treasure room that would have been generated, minus the item and room textures.
If the Treasure Room would have been a double Treasure Room, the Devil Treasure Room will also have two items. Unlike the double Treasure Room, however, both items can be taken. This also goes for Treasure Rooms on the alternate path (the blind item will still be blind).
This trinket also converts Treasure rooms on the current floor when picked up. However, this only affects Treasure Rooms that have not been visited yet.
The same goes vice-versa: dropping the trinket will convert red treasure rooms to normal ones unless they've been visited.
Items bought in the Devil Treasure Rooms will affect Angel Room chance like a normal devil deal would.
When entering floors during the Ascent, generated and open Treasure rooms will keep their previous state from the descent, which can not be affected or changed in any way.
However, unvisited Treasure Rooms can still be converted by dropping and picking up the trinket.
Does not affect the Silver Treasure rooms in Greed Mode.
Extra treasure rooms generated by Red Key will be affected normally.