White Soapstone Summon

As an additional cost to cast this spell, tap a creature you
control.

Search your library for a creature card with mana value
less than the tapped creature's mana value. Put that card
onto the battlefield, then Shuffle. It gains haste and "at the
beginning of the next end step, shuffle this card into it's
owners library."

In Lordran, the flow of time is distorted, and the White
Sign Soapstone allows Undead to assist one another.