I've just started a NG+ game, and killed the Old Dragonslayer in Heide's Tower of Flame. Just before the boss there is a Red Specter, he carries a lance of a sort, that was extremely annoying, but very cool! It does a series of stabbing/lounge-like attacks that are very hard to dodge.
What weapon is it? And is it available in-game?
It may have been NG++, I burned a bonfire ascetic in the area on NG, but I am guessing he spawns at NG+.
Best Answer
In this reddit post below it seems that a player did indeed find a phantom in NG+ and kill it receiving a skirt from the enemy.
Quote from the poster Poo_Tsunami:
"I found a painting guardian in Aldia's keep! Probably old news but I haven't seen this mentioned anywhere so I thought I'd take a picture of the item it dropped and the description. I didn't have time to snap a picture of the actual phantom because it was trying to kill me.
From the back http://imgur.com/AtAljz1
From the front http://imgur.com/18K6T4m
The description http://imgur.com/TwZ4ZGv
Although the description makes no mention of guarding paintings, the armor is exactly the same and the phantom was hiding behind a painting dual wielding square blades."
http://www.reddit.com/r/DarkSouls2/comments/210n3h/ng_red_phantom_possible_spoilers/
So yes it may be possible to obtain the lance he was using.
There is a list in the link below that tells you how to acquire each regualar lance in the game. If it was another normal players lance he was using you should be able to identify it. http://darksouls2.wiki.fextralife.com/Lances#.U1l-U_ldXT8
(I'd also like to note that this is NOT one of the npc invaders.) List of NPC's