The soulflame horn is a great idea! It's a nifty piece of kit for team bossing. However I do see it sliding into being a rarely used item or very cheap bit of kit due to the sacrifice of an inventory slot.
My suggestion, make it a 3-tick melee weapon. It's not smacking big numbers with those stats anyways, but will increase it's utility in cases where 3-tick weapons are useful such as TOB nylo room and Judge of Yama.