Hey, i just noticed that Lord of the heavenly prison has a huge problem which led to some frustration on my side:
During your Main Phase: You can activate this effect; this card in your hand becomes revealed until the end of your opponent’s turn. While this card is revealed by this effect, Set cards on the field cannot be destroyed by card effects. If a Set Spell/Trap Card is activated (except during the Damage Step): You can Special Summon this card from your hand, then, if you activated this effect while this card was revealed, you can reveal and Set 1 Spell/Trap directly from your Deck, but banish it during the End Phase of the next turn. You can only use 1 “Lord of the Heavenly Prison” effect per turn, and only once that turn.
If you have 2 or 3 of him in your hand, you can’t tell which one was revealed by his effect.
So if you use his 2nd effect in your opponents turn, which lets him special summon, you might pick the wrong one and don’t get the third effect which lets you set a card from your deck.
I wrote this in discord and got told i should post it here. My suggestion would be, that the one which is revealed by his effect, should somehow visually be marked so that the player can see which one is revealed to your opponent and would not pick the wrong one later on.