The Yamato Perpetual Reactor is a device found in Shin Megami Tensei IV.
A machine which the Ashura-kai uses to supply Tokyo with electricity. Yuriko and the Ring of Gaea are attempting to acquire it for themselves, leading to Tayama sending the Samurai on a mission to kill her to stop them. It also holds the power to open portals to alternate worlds and even the Expanse, the realm from which demons originate. An NPC mentions that it used to be owned by the military and that rumors spread of it being used to summon demons or it being a plasma weapon. It is located in Camp Ichigaya.
In the ending where Flynn sides with The White, it is faced as the final boss. However, as it is not alive, it has no defenses or attacks and can be destroyed by just attacking it, making it malfunction and release a black hole across the universe, destroying everything and freeing the world from suffering.
It is referred to as "Great Abaddon" by Merkabah, referencing Abaddon's similar role in Shin Megami Tensei II. In the Law ending the reactor is activated by Merkabah, creating a black hole that swallows Tokyo and all of its inhabitants, including Flynn and Merkabah. The reactor can be used on the Chaos route at any time to return to Infernal and Blasted Tokyo, as well as after defeating Lucifer on the Neutral route.