Equip Excalibur on Mont Leonis

Does it do anything special for him?

No. This sword would just be a piece of garbage after you've gotten other farmable MR swords etc.

I completely disagree. I prefer Excalibur over my Nagnarok for pvp with Englbert. Less damage, but high ACC/crit which is great against the evade meta.

Also means I can use the second equipment slot for a something like mythril helm (10 evade), rather than alexandrites ring. Which makes him even tankier than before, while still having a decent damage output.

It all depends on your situation.

You can clearly notice it is the main weapon of Gilgamesh. The color, the shape, all match