Its a stellar two drop
Comes with the outer body as well as a "lifter" to go underneath your cards or divide them
Some self-mill examples might be Sidisi, Brood Tyrant, Grisly Salvage, or Otherworldly Gaze
Thankfully, over the past decade, even Wizards has realized that partners are pretty broken, and now they avoid printing more cards with the mechanic, instead relying on various partner-with-style mechanics that do something similar but with a much smaller pool of cards to choose from, like how you can partner a Doctor's companion with a Doctor from the Doctor Who set
Its high mana cost means its typically relegated to the late-game, but pitching it early for a basic land is more valuable than youd think in a deck without access to greens ramp spells
It existed because there were problems at the time dealing with getting mana from spells in time to be able to cast the spells