From Diablo Wiki
Reflects Damage is a Boss Modifier possessed by Champions and Bosses (but not boss minions) on Nightmare and higher in Diablo III.
The property works as a sort of Thorns effect on the boss, causing the hero to take back as damage some percent of the damage dealt with attacks. The reflected damage comes from all attacks, not just direct one. Ranged attacks, and even spells such as Hydra or Sentry. The effect can be seen most clearly for players who have health bar displays enabled, as your hero will spout a steady stream of red numbers from all the reflected damage you are hurting yourself with.
- Monster Level Minimum: 31
- Available to: Champions and bosses.
- Damage Type: Physical.
- Additional Resistances: N/A.
The v1.0.4 patch in August 2012 fixed a bug related to this affix.
In December 2012 Blizzard revealed that they agree with the numerous player complaints that the modifier is too damaging and that it will be changing in an upcoming patch. 
2) After seeing a number of posts from players about RD dealing what they felt to be TOO much damage (especially from minions), we did some additional testing and discovered that Rare minions with the affix were in fact doing more damage than intended. This is an issue we’re looking to address in an upcoming patch.
I personally have no problem with the way it is now but if it had been tested it could have been polished a bit better then it is now.Lylirra: We were honestly pretty happy with the way RD functioned prior to 1.0.5, and thought it was in a good place in terms of balance. That changed, of course, whenever we implemented Monster Power and reduced defensive bonuses in 1.0.5. After reviewing player feedback post-patch and reexamining the affix as a whole, we decided that it could use some improvements. Overall, we think the affix will be much more manageable (if not potentially more enjoyable) as a result. That doesn’t mean our earlier opinion was wrong, though, it just existed under a different set of conditions.
This property is most dangerous to glass cannon type characters who do not have high regeneration or healing or life leech, but it can be deadly to any character who doesn't have enough healing, usually in the form of life on hit or life leech, to offset the incoming damage.
Ironically, Reflects Damage becomes more dangerous to very powerful characters. For instance, a moderately well-equipped character might deal 100k DPS with 45k health. As that character gets better equipment, their DPS would rise to 200k or more, while their hit points and regen stayed roughly the same.
A good strategy is to avoid hitting all of the Champions, or the boss and his minions, at once. Hitting a single target shouldn't be so deadly; it's when you are dealing big AoE damage to several of them at once, and getting the reflected damage from all the monsters. This is tough, since most of the best attack skills in Diablo III are great specifically because they hit so many enemies at once.
Reflects Damage does not grant any appearance change to the boss or champion with it. The property is seen only in the tooltip hover display, or by the effect on the player, as a steady stream of red damage taken numbers will fountain out from your hero as you deal damage to the enemies.
The effect is best seen in video, as a shower of red numbers constantly sprouting out of the hero.
A Demon Hunter shows a brilliantly-effective cheese against Siegebreaker on Inferno (when he gains Reflects Damage as a property), using a shield with high life regen to negate the reflected damage entirely.
|Monsters of Diablo III Beasts Beasts: Superunique Demons Demons: Superunique Humans Humans: Superunique Undead Undead: Superunique Objects Bosses Boss Mods Monster Info Fanmade|
Bat - Beast - Blood Hawk - Demonic Hellflyer - Electric Eel - Khazra - Lacuni - Lamprey - Quill Fiend - Rockworm - Sand Monster - Sand Shark - Sand Wasp - Scavenger - Spider - Spiderling - Swarm - Withermoth - Wood Wraith -
|Primary Quest Uniques||
Act One Bosses:
Skeleton King -
The Warden -
Strong CC (Limit 1): Knockback -- Nightmarish -- Vortex -- Jailer
|Act One Uniques||Araneae - Web Spit||Leoric - Summon Skeleton, Teleport Strike, Triple Cleave||The Butcher - Smash, Grappling Hook, Spear, Frenzy Charge|
|Act Two Uniques||Maghda - Forcefield, Summoning Ritual, Punish, Moth Dust, Teleport.||Zoltan Kulle - Fiery Boulder, Time Bubble, Ceiling Collapse, Energy Twister,Vanish, Creepy Laugh||Belial - Phase One: Triple Fireball, Summon Vipers. Phase Two: Poison Meteors, Ground Pound, Lightning Strike, Lightning Breath.|
|Act Three Uniques||Ghom - Acid Slime, Sneeze, Gas Cloud, Chomp.||Siegebreaker - Triple Hit, Charge, Leg Stomp, Grab and Throw.||Cydaea - Pain Bolts, Web Trap, Spider Pits.||Azmodan - Bellyflop, Laser Attack, Falling Corpses, Demon Gate, Pool of Destruction, Globe of Annihilation.|
|Act Four Uniques||Rakanoth - Cleave, Teleport Strike, Blade Strike, Volley, Summon Minion.||Izual - Frost Nova, Frozen, Summon Minions.|| Diablo - Phase One: Claw Rip, Charge, Hell Spikes, Shadow Vanish Grab, Curse of Destruction, Ring of Fire, Ground Stomp, Curse of Anguish, Curse of Hatred. |
Phase Two: Shadow Clone. Phase Three: Overdrive, Lightning Breath.