Sliver Gravemother is a mythic Legendary Creature — Sliver card from Commander Masters (CMM), collector number 777. Current market price: $29.17 (foil $45.74), TCGplayer market prices via Scryfall.
The "legend rule" doesn't apply to Slivers you control. Each Sliver creature card in your graveyard has encore {X}, where X is its mana value. Encore {5} ({5}, Exile this card from your graveyard: For each opponent, create a token copy that attacks that opponent this turn if able. They gain haste. Sacrifice them at the beginning of the next end step. Activate only as a sorcery.)