It is the weapon you get when you defeat Witherzilla.
Attack:from 30000 to 100000 + enchanted + debuffs
From the forum:
☀The largest weapon to ever be wielded by a mortal, this puppy is sentient in it's own right. It does up to infinite damage to entities in your range and ignores all forms of restrictions. It's also smart: when you hit your first entity with this weapon, keepInventory is set to true so you never lose this awesome weapon, and it can tell you of nearby mobs and ores with right clicks: shifting for mobs, not shifting for ores. It can't break, but also can't be crafted. I'll leave it up to you guys to figure out how you get this in survival. That's something for you to figure out. Until then, have plenty of fun with it in creative (if you're worthy for even that). No one will judge you (except the blade). Just don't complain on how it can't kill some of the Titans cause it's explained in this forum.
If the Optima Axe is also in your inventory, then the weather will be locked to thunderstorm, and lightning will automatically attack every mob.
It Does 2125 Damage on 0.475 30000 on 0.490 and 100000 on 0.495
Use this command if you have access to commands to get the Ultima Blade in old 1.7.10 versions:
/give (name) thetitans:ultima_blade 1 1