Skeleton Mages are a rare variant of skeleton that make use of Magic to attack. Thus, it is recommended to wear magic-resistant armour such as dragonhide armour. They play a role in the Swan Songquest, where the player creates an army of them to help defeat the sea trolls.
I infect your body with rot...
The sound when a Skeleton Mage successfully weakens a player.
Level 16 mages usually attack with melee and will occasionally cast a spell, exclaiming "I infect your body with rot..." that resembles Weaken as a projectile and Curse when it lands successfully. If successful, it will lower the player's Attack, Strength and Defence stats by 4 levels.
Level 83 mages cast a spell that resembles Fire Strike and will sometimes use Vulnerability on the player.
Malignius Mortifer will occasionally spawn a non-attackable Skeleton Mage, which follows him around for a while before disappearing. The player can also summon their own variant using Bone seeds after the completion of Swan Song.
During the cutscene where the Skeleton Mage army attacks the Sea Trolls, they instead have a combat level of 64 and attack using spells that resemble Earth and Fire Bolt.
Interestingly, if the player opens bone seeds not for the purpose of defeating the sea troll army, the Skeleton Mage that appears takes on a different appearance, appearing identical to the ones found in the Waterfall Dungeon and the NPC variant that Malignius Mortifer can summon.