The battle robe bottom is a reward that can be purchased with tokens from the Fist of Guthixminigame. It degrades with use similar to the Barrows equipment and costs 100 FoG tokens to repair, however it may only be traded when completely degraded. It requires level 50 Defence to wear
Similar to all other armours obtained from the Fist of Guthix minigame, it can be degraded for trading purposes by talking to Reggie in the cave and asking him to degrade the item.
When used with a staff of light, and a full set of battle robes, there is 50% chance that you will not use any runes when casting a combat spell.
After a total of 6 hours of combat the robes will be degraded (from 100% to 0%). This won't be affected by the amount of runes the robes save up. (Said another way: it is only time in combat that degrades the robe - not the amount of runes saved.) This means that using the staff of light will not prolong the lifespan of your robes. This may well affect the reason of buying the robes - be it on the Grand Exchange or from Reggie.
Nevertheless, players are advised to get this item because you gain the highest profit from this item.