Selfless Spirit

The trick with this Spirit is that the protection is free in mana but costs you the protector. Sacrificing it grants every creature you control indestructible until end of turn, which means a single white two-drop can blank a board wipe, win an unfavorable combat trade, or push a creature through a removal spell, all for the price of a 2/1 flier that has already attacked. The activation has no mana cost, so it stays live at instant speed regardless of what you tapped out for, and an opponent has to play around an answer that demands nothing from your hand or your mana. The flying body earns its keep in the meantime, chipping in real damage in the turns before you cash it in rather than standing around as a held-back insurance policy. Against sweepers it does the structural work of a Boros Charm or a cycle of indestructible-granting effects, but folded into a creature you wanted to play anyway, which is why it became a reflexive inclusion in white aggressive and tokens decks: curve filler that doubles as a built-in trump against the exact card those decks fear most. What keeps it fair is the one-shot nature of the shield; once you spend it, the board is mortal again, so the card asks you to time the sacrifice for the swing that actually matters.