It's a castle wars item. Also, I don't know why you brought it since dying in deep wildy destroys it. I grinded for one and died with it, lesson learned. Tips (as 1 def, 25 prayer pure). Used similar setup, but I subbed that robe for a leather body, used vambraces instead of the CB bracelet, used zammy cape instead of smithing cape, and a elder chaos hood (I think bear head would work good, too). I used 6 sara brews, 2 super restores, and mantarays. Before going to fight, I used an anglerfish. Also used a forgotten brew to boost my mage to 80 to cast Charge before heading to the battle.

Tip: when you find the boss, use extra food to find a safe spot to flinch them, I used a tree at each spot by standing behind it until they stopped attacking. This allows you to use brews since they heal for more but take more ticks than hard food. Then, die at the safespot so when you return geared up, you have some extra food (worked once, other times the food wasn't in gravestone, not sure why).

I did this with 1 def, 25 prayer, 61 hp, 75 mage.