Sold by this NPC: http://classicdb.ch/?npc=11278

You need to complete the Scholomance quest line given by this NPC http://classicdb.ch/?npc=11216 outside Scholo to receive an item that allows you to see the vendor.

You can only see Magnus after finishing the chain of quests (I believe there are three). The quest is begun by Eva Sarkoff who is a ghost standing outside Scholomance with her husband. There is no indication that she will give you a quest, you must just walk up to her and start asking her questions. All of the quests are inside Scholomance and they are not easy. You will have to go back outside after you finish each quest to get the next part of the chain. After you finish the last quest you will receive [Spectral Essence]which when applied will allow you to see the ghosts in Caer Darrow. Magnus is in Caer Darrow itself inside the Smithing building.

He sells the following items: (bind on pickup)
- Plans: Huge thorium battleaxe (level 280) 2g
- Plans: Ornate thorium handaxe (level 275) 1g 60s
- Plans: Ebon Shiv (level 255) 1g
- Recipe: Major Mana Potion (level 295) 3g
- Recipe: Transmute water to air (level 275)1g 50s
- Plans: Storm Gauntlets (level 295/ARMORSMITH)4g

I finished these quests with my 59 pally, with a 59 mage, 60 mage, 60 priest and 60 pally. All of these plans & recipes require basic materials - and other than the storm gauntlets, which are blue, the weapons are green items.