Necrosma
Prism Pokemon
Prism Armor as ability
Prismatic Laser as attack

"Light is apparently the source of its energy. It has an extraordinarily vicious disposition and is constantly firing off laser beams"
"Reminiscent of the Ultra Beasts, this life-form, apparently asleep underground, is thought to have come from another world in ancient times."

Lumiose City is called "The city of Light"
Lumiose has Prism Tower
Kalos is the region with the most known ancient lore

Necro means "a combining form meaning “the dead,” “corpse,” “dead tissue,” used in the formation of compound words: necrology"

Xerneas Life Yveltal Death Necrosma Life form beyond death?

Zygarde in Alola, Necrosma in Kalos exchange?