If the battle is not finished after a few minutes, bosses can do a variety of things, including buffing the boss' damage, or debuffing the player. The effects will increase until the battle is ended. The time and effects vary by boss, and not all bosses have them. The idea is making the combat harder after a certain amount of time, up to the point where the battle becomes unreasonably hard for an adequately geared player. A general rule of thumb to keep in mind is to try and keep on damaging the boss monster most of the fight. The reason for this is twofold. Firstly, once the boss monster is killed, if it had any minions summoned, they will prove to be of little to no threat afterwards. Secondly, for most adequately equipped players, most of the attacks used by the boss monster will prove easily survivable, especially if they have a high degree of endurance and healing compared to the difficulty level.
There are two types of enrage timers: hard and soft. Hard timers have a set time which makes the combat much more difficult (though not outright impossible) after X seconds. Soft timers gradually increase the difficulty of the fight over time, and continue to do so until that encounter ends, be it with the death of the Nephalem, or the boss.
See individual boss pages for timers (mostly 3 or 4 minutes) and effects of the enrage. Known effects include:
- Covering the entire battlefield with harmful effects that at some point will generally be unavoidable without the proper immunity amulet for the encounter (Ghom, Belial, Adria)
Some bosses, most notably Diablo and Malthael, do not have enrage timers. This is due to these boss fights being complicated enough, and these bosses having much larger Life pools, and therefore players are allowed to fight them for as long as needed. However, unless managed properly, they certainly can put the player in a situation similar to hitting the enrage timer, such as Malthael spamming his Death Fog.