Rhiannon

Vigilance
As long as you control 5 or more lands, Rhiannon
and land creatures you control get +2/+2
{g}, Sacrifice a creature you control: Target land
you control becomes an elf creature with power
and toughness equal to the power and toughness
of the sacrificed creature. It's still a land. (Activate
this ability only any time you could cast a sorcery)