Antimagic

Version 0.15: This article may not be up to date for the latest stable release of Crawl.

It disrupts the flow of magical energy around spellcasters and certain magical creatures (including the wielder).

The antimagic brand causes weapons to interfere with the spellcasting of anything wounded by them. Monsters struck by these weapons gain a temporary spell failure rate (this rises as they take antimagic damage but falls over time), while players simply lose MP with each strike. This effect works on anything that uses magic, including silence-proof targets such as demons and orbs of fire.

Unfortunately, so long as you are wielding this weapon, your max MP will be reduced by 2/3, making it something that caster characters should be cautious about wielding carelessly. However, a character who has already exhausted his MP pool is free to switch to it mid-combat, and in some cases the effect may be worth the sudden loss of the majority of your casting capacity.

The antimagic brand is Trog's special brand. His weapon gifts often (but not always) carry this brand. Vine stalkers also possess a unique antimagic bite attack which functions similarly.

Desirability

In spite of its inability to deal extra damage, antimagic weapons are extremely useful, at least in certain situations. A well-enchanted weapon in the hands of a capable fighter will be able to cause most of an enemy's spells to fail, which amounts to semi-paralysis. As such, antimagic weapons can reduce the threat posed by powerful end-game opponents such as the lords of Hell or orbs of fire dramatically; you just have to get into melee range with the enemy first.