Enchantment - Saga Sheoldred, Whispering One Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.) At the beginning of your upkeep, return target creature card from your graveyard to the battlefield
Mana and card draw are the most important resources in Magic, so getting both from one trigger plus whatever rewards you get for sacrificing your creatures is a massive advantage
There are a few staples that any Elf deck will love like: Elvish Archdruid Elvish Warmaster Overwhelming Stampede There are several high-power spells in the deck too, like Heroic Intervention and Asceticism to keep your creatures safe, and Genesis Wave to turn your mana into even more creatures
May be a great opportunity for you to draw an clutch of cards, or smoke an opponent with a board full of tokens
The original art book in 1998 sold terrible
It's annoying and pervasive, but I don't think it rises to the power of the Top Ten, because, well, Slivers are fun, right