A true berserker is a player character who fights in berserk mode naked. This provides a substantial increase in to-hit and damage on top of the regular Berserk bonus.
The slots that need to be free of any items are the headgear slot, the body armor slot, the girdle slot, the cloak slot, the gauntlets slot and the boots slot. Bracers, rings, amulets, missile weapons and ammunition, tools, shields and of course melee weapons can be worn while true berserking.
Body armor with the "of rage" suffix can be worn without denying the true berserker bonus.
The PC must be using at least one proper melee weapon to be a true berserker; unarmed combat will not benefit, and unconventional melee weapons such as large rations or torches do not count towards meeting this requirement. Monks cannot true berserk under any circumstances.
If the requirements for true berserking are met, every melee attack will yield the "You lust for blood!" message before hits or misses are displayed. In ADOM versions prior to 1.2.0, moving into non-hostile NPCs will not trigger a "Do you really want to attack..." message while true berserking — the PC does not stop to think and instantly attacks. This may prove to be fatal for non-hostiles that are not immediately distinguishable from regular monsters, such as the ratling dealer, though it does not immediately become your problem.
To-hit and damage are increased by an additional 6 each, which lets even characters with low Strength scores and pitiful weaponry kill early game monsters with ease and impunity. This is especially attractive to PCs that do not possess useful armor to begin with, such as many Healers. It should be kept in mind that being naked and a true berserker does not impede the character's ability to become a Coward if necessary, as the combination of low DV and non-existant armor is quite dangerous against ranged attackers or greater numbers, even if the PC can kill a monster pretty much with one attack. However, for extended periods of defensive tactics, equipping any PV-granting items the PC may have is highly recommended.
Missile attacks do not benefit from being a true berserker, though they do get the regular Berserk bonus.