Revenge is a Might Barbarian skill unlocked at Level 13, which hits all enemies in an AoE around the Barbarian and heals him. Only usable if the Barbarian's been hit.
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The Barbarian retaliates by smashing the ground with a vengeance, taking the life force of enemies who still cling to life.
The following is a very quick summary. See the Revenge rune effects page for a more thorough description of rune effects, or any of the individual rune pages for numbers, screenshots, blue quotes, and much more.
Revenge is a good skill to heal up when swarmed, as when surrounded by monsters, the chances of Revenge being usable becomes quite high. The amount healed is in percentages, so hitting 5 enemies will give you 10% of your total health at rank 1. The damage is also good with 150% of a very damaging yet slow weapon will make this skill useful to cause some havoc and heal up.
The following Barbarian passives provide a benefit to Revenge, runed or not:
Revenge was first introduced in the BlizzCon 2008 demo as a Tier III skill in the Juggernaut Skill Tree, which did 100% weapon damage. The BlizzCon 2009 version was pretty much the same, but with increased damage.
Revenge was one of the preset skills for the Barbarian in the BlizzCon 2010 demo. Flux had this to say about its functionality:
What happens with Revenge is that you put it on the 1-5 keys, and it stays grayed out/unusable until you get into a fight. At some point during the battle, if you have enough Fury and you get hit, it activates. Then you can hit the hotkey to use it, and your Barb makes a kind of spinning back swing, like a counter in a fighting game.
...I didn’t actually need it during the demo with the Barb; I was never low on hit points and could have passed up 75% of the health globes I earned. But the whole game won’t be as easy as this demo, and with more points in Revenge, or perhaps with skill runes, the life leech might be raised (or the activation might be increased in frequency) up enough to make this a really valuable source of health.
Skill tiers were removed for the July 2011 Press Event, allowing skills to unlock steadily from levels 1 to 30. Consequently, all skill levels were adjusted and Revenge became a level 12 skill.
The Diablo III Beta started in September 2011, and while it ran, all of the skills continued to undergo extensive iteration. Skill changes varied from changing the levels the skills are unlocked, to changes in resource cost, damage, and even function. A few skills were removed altogether, and a few new ones were added. Also, the entire skill system received an overhaul with the Beta Patch 13 update in February 2012, with physical runes replaced by rune effects that automatically unlock at various levels for the different skills.
Revenge stayed at level 12 when the beta started. With the Patch 13 changes, it was classified as a Might skill and bumped up to level 22. For the Beta Patch 14, it was dropped back down to level 13.
See the Barbarian skill archive for more details on previous versions of Revenge and other Barbarian skills.
The mechanic for this skill was probably inspired from Warrior skills in World of Warcraft, where you can only cast some skills if you've been hit in the last X seconds. With the restraints on the use of the skill, it's not strange that it is Fury free.
Revenge can be seen in action on video here:
You can find pictures in the Diablo III screenshot and picture gallery: