The warlock legs are a holiday reward from the 2008 Hallowe'en event. It has no bonuses, and is part of the warlock costume, which also includes the warlock top and the warlock cloak.
It is no longer possible to obtain the warlock legs, or any other part of the warlock costume. During the 2008 Hallowe'en event, players had to go trick-or-treating in Rimmington, Port Sarim, and Draynor Village to receive candy from certain NPCs. For this part of the costume, five wrapped candies needed to be received. Maggie would give you the item when you had all the wrapped candies.
Female characters cannot wear the warlock legs, or any other part of the warlock costume. Female characters would receive a witch costume, and if they try to put the warlock legs on (if they changed their gender), it will turn into a witch skirt. If a male character tries to wear a warlock legs while changing gender, it will also turn into a witch skirt.
In the members-only Costume Room of a player-owned house, the warlock costume can be stored in a toy box.