Weak
Version 0.14: This article may not be up to date for the latest stable release of Crawl.
Weak is a status effect which reduces the target's melee damage output until sufficient time has passed: in essence an anti-might. It reduces monster melee damage by 33% (exactly cancelling out monster might), and player melee damage by 25%.
At present, the main sources of weakness are the attacks of orange demons and Elyvilon's wrath upon killing an apis.
In addition, melee attacks with the demonspawn mutation "Black Mark" can also have this effect on monsters.
You can immediately end weak through Elyvilon's Purification ability.
History
Weak was added in 0.13