The Massive Possessor is a type of Pureblood Heartless that features in Kingdom Hearts 358/2 Days. It is made up of multiple Possessors that have all come together and merged into this large creature.
Roxas encounters Massive Possessors in both Wonderland and Beast's Castle. They are quite weak and more of an annoyance than a genuine threat, but after being depleted of HP, they come apart as multiple small Possessors that can swarm Roxas if he is not careful.