>>58541320If don't have the teraleak pic, just this one (hope someone can post it) but the Ultimate Weapon has two parts, the flower that gives life and the thorns that take it, the former is the main cannon you see in Geosenge Town, which was originally used to give Floette eternal life, and the later is the megaliths from route 10 which powered the machine by absorbing life from countless pokémon.
Likewise, Xerneas is the one who gives while Yveltal is the one who takes, so it makes sense to employ both legendaries, the one who gives as main battery, supplying energy to the machine, and the one who takes as absorption device along the megaliths. If anything this means that while you wake up Xerneas so it stops giving its energy to the weapon in some other part of the hideout they also had Yveltal draining energy for the weapon as well, that also would explain why Lysandre could fire the weapon despite losing one of the legendary pokemon.
Plus, Lysandre being alive and immortal in Z-A matches the usage he gives to the weapon in Pokemon X. In Pokemon Y he doesn't try to make himself and others immortal after you beat him, but to nuke everything. Pokemon X is clearly the canon version of the events.
