Rayquaza
The legendary sky dragon in its Mega form. Found atop the Sky Pillar after a challenging climb involving trainer battles and puzzle solving. When you catch it, since the only requirement for rayquaza to be in Mega form is to know Dragon Ascent, you can able to switch between the Mega and non-Mega form of Rayquaza
Locate Structure Command
/locate structure cobbleverse:legendary/sky_pillarHow to Obtain
Materials Checklist
Craft the Mega Rayquaza Radar to locate the Sky Pillar. Materials can be purchased from the Department Store:
For the Mega Stone, you'll need to find a Megaroid structure underground (Y-level -20 to -32). Mine the Keystone Ore inside with a diamond pickaxe or better. Use `/locate structure mega_showdown:megaroid` to find one.

Use the radar to locate the Sky Pillar. It's a massive tower structure you'll need to climb from the inside.

Upon entering the Sky Pillar, immediately turn left to find a hidden barrel containing Tera Shards.

After climbing the first ladder, you'll face Leader Ivan who has a Level 75 Primal Kyogre.

Victory rewards you with the Primordial Gem. Hold onto this - you'll need it later for the Sky Core.

In the back right of Ivan's room, dive under the water to find another hidden barrel.

Climb another ladder to face Leader Max who has a Level 75 Primal Groudon.

Victory rewards you with the Desolate Gem. Keep this safe for the Sky Core.

Before climbing the next ladder, check the right blast furnace to find a Fire Gem. There is probably more loot, I was too lazy to explore the big lava pit in the middle

Climbing yet another ladder, you'll face Lyris with a team of dragons. View the full team at the Progression Guide here

Victory rewards you with the Aether Gem. That's three gems collected!

Break the back of the blue dragon statue's mouth in Lyris's room to reveal a hidden chest.

Climb to the puzzle room. Behind a small ladder, look up to spot a hidden lever.

Flip the lever to open a hidden block above the chest in the back, revealing a secret barrel.

Inside the barrel is the Emerald Emblem - the final ingredient for the Sky Core. You now have all four components! Check JEI for the crafting recipe or look at the next step

Materials Checklist
Combine all four items to craft the Sky Core. Place them in the crafting grid as shown.

Climb the final ladder and use the Sky Core on the altar to summon a Level 75 Mega Rayquaza. Bring your strongest Pokemon - you'll need to survive some hits while trying to catch it.

Bonus: A Shiny Level 100 Rayquaza spawns in The End as soon as you enter through the portal! Rush to the center quickly or it may despawn. Bringing a flying mount is highly recommended.
