The infinity bottoms are a magic armour item obtained from the infinity robes set worn in the legwear slot. 50 Magic and 55 Defence is required to wear the infinity bottom. The infinity bottom can be obtained as a reward in the Mage Training Arena minigame by giving the Rewards Guardian 450 telekinetic, 500 alchemist, 5,000 enchantment, and 450 graveyard pizazz points without any Desert tasks discount.
A dragonbone upgrade kit could be combined with infinity bottoms to make dragonbone mage bottoms. This is a cosmetic enhancement and does not change the stats in any way.