Plaguing Nightmares

Indestructible, protection from instants and sorceries
Plaguing Nightmares must attack each turn, if able.
Creatures you control may block Plaguing Nightmares.
Whenever Plaguing Nightmares is blocked by a creature you both own and control named Fort of Resistance, exile them, then meld them into Raeib, Birth of Plagued Dreams.