A large 3x3 craft-able block, requiring numerous complicated recipes using endgame items in bulk. An example of the components required to craft it could be "Ultimate Weapon Petals", "Ultimate Weapon Core", "Ultimate Weapon Focus", etc. When powered by redstone, the structure could glow a bit too to make it look cool.
Once the weapon is completed, the builder of it can right click the weapon to tie themselves to it. From there, the weapon will track how many pokemon the player kills/catches. The first stat to reach 350 since crafting will "power" the weapon and cause it to glow red or blue depending on which legend the player went for.
350 kills will net the player Yveltal, of course.
and 350 Catches will net the player Xerneas.
Once the player has amassed this total and right-clicked the Weapon, it will shoot a coloured beam into the sky before spawning the respective legend on the player.
Another function of this weapon, similar to the Ilex Shrines function with Pichu, could allow it to transform a regular Floette into an AZ's Floette when thrown at the structure if the player has amassed 350 kills/catches. Floette will undergo a small lil animation at the structure, consuming the player's current tally and turning it into an AZ Floette and prompting the player to replace one of it's moves with Light of Ruin, if all 4 slots are filled.