Personally, I think best of all time is the Shields system from Duel Master.
Each player puts five cards from the top of their deck face-down at the start of their game; these are their Shields. Whenever a player takes damage, one of their Shields breaks and goes into their hand. Many cards would have “Shield Trigger” effects that would allow them to be played after entering your hand this way. Some, such as Holy Awe, could turn the tide of the match.
Does a videogame count? In Inscryption, when you lose a run you’re able to make a custom card by combining three of the cards you collected in that run. After doing this several times you can make a ridiculously OP card that makes the run a breeze.
Inscryption is amazing, and I’m not even a card game person. Fuck that damn egg deck, though.
I also choose Inscryption but for the ability to sacrifice cards to add their ability to another card, which can also get ridiculously OP.
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For example, if you put the Cockroach’s Unkillable Sigil on a Skink, whenever it dodges an attack it creates an Unkillable Lizard Tail card that’s free to play and can be sacrificed repeatedly, as well as used to block. This is one of the ways of dealing with the Kaycee’s Mod challenge that makes the bosses spawn 8 Grizzlys.
There’s so many weird combos and stuff to discover in the game, even after completing all the challenges in Kaycee’s Mod I wish there was more to do.
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The wires and gems mechanics are really cool too. Honestly, the felt kinda underutilized, but I guess they’re both towards the tail end of the game, and both fairly complex.
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I’m stuck at the Angler currently