So, I found that if you play Adaptive Amalgam, Silver Vanguard/Flesh Behemoth and they die. If you then play Da Undatakah with Spiritsinger Umbra already on the field.
The game basically goes through a bunch of interactions were, he’s sent back into the deck with all his enchantments, and then resummons himself typically 20, 30, times. It skips through both players turns while that happens.
If you close out the game, I’m assuming it works for both sides, but I can only see my own. The interactions don’t show and the other player is basically forced to watch the interaction while I can still interact with the game normally.