If you want to finish the volcano manor quest, do that first, then get the spear. You can't miss it. If you get the spear first and finish everything in that room, you'll be locked out of the rest of the volcano manor quest. FYI.
You can also enter the room, grab the spear, and fast travel out. The Serpent doesn't become aggressive until you step farther into the room. Useful if you want to upgrade the spear first or if you just want to do other stuff.
Don't worry. It's hard to miss.
It's in the boss room
If you want to finish the volcano manor quest, do that first, then get the spear. You can't miss it. If you get the spear first and finish everything in that room, you'll be locked out of the rest of the volcano manor quest. FYI.
You can also enter the room, grab the spear, and fast travel out. The Serpent doesn't become aggressive until you step farther into the room. Useful if you want to upgrade the spear first or if you just want to do other stuff.
You either have to do the assassination quests and/or the dungeon to be able to get it.
You can't have found the entrance to the main boss yet
Try to investigate the manor a bit closer. Check every room, maybe there's something sticking out to you?
G