Sunken Atlantis

Hexproof.
All Merfolk you gain control gain +1/+1.
{u} Target Elf, Human, or Goblin you control
adds Merfolk to its type and adds blue to its
colors until it leaves the battlefield.
There are many sunken cities, but Atlantis is
special. It is seemingly origin of all true Merfolk,
who retained some ideal of it in most of their
oral histories. - Commodore Guff