So... I was just pondering odd and unusual card rules and I decided to design a mana dork for all of you Melvins out there.
Creature- Elf Druid (U)
, Return Elvish Immortal to it's owner's hand: Add . If this mana is spent on a creature spell, that creature enters the battlefield tapped.
This relatively complex mana dork (designed for something like Modern Horizons) is designed to be what it says on the tin, a nigh-unkillable elf (assuming summoning sickness is gone). Self-bounce as a cost means that it cannot be responded to and the fact that it is for a mana ability means that even cards with split-second can't hurt it.
While infinite combos are technically possible, I feel that requiring both amulet of vigor / intruder alarm + a haste enabler presents enough hoops for the payoff of an infinite storm count (or infinite mana if you also add a cost reducer).