- Potions of might.
- Berserking includes a might effect.
- The Demonspawn mutation Powered by Pain can cause a might effect.
The +1d10 damage is rolled separately from the effective enchantment roll (slaying + to-damage of your weapon), but the defender's AC reduction is still only applied to the total damage (i.e. it is not applied separately to the damage from the might effect). This means that as compared to +10 slaying, might gives a more consistent damage bonus that is also very slightly higher on average.
Tips & Tricks
Might is useful for dealing with monsters that are troublesome, but for which berserk or Haste would be overkill and/or unsafe - compared to those, might has no drawbacks unless you extend it, so it's safe to use. Unfortunately, as it is only available through a potion or by going berserk, you may find yourself with a very limited number of uses per game.
This effect is usually far stronger for monsters: it multiplies their damage output by 1.5, instead of simply adding 1d10. For this reason, one should exercise extreme caution around monsters such as moths of wrath, deep troll shamans, Rupert, and Wiglaf.