What is Reborn in Hearthstone?
Hearthstone has many card effects that are common enough that they get their own keyword - a word or two that immediately tells you how the card works. Let's take a look at the Reborn keyword.
Resurrects with 1 Health the first time it dies.
Reborn is a keyword that appears exclusively in Year of the Dragon, appearing in Saviors of Uldum and later making a few appearances in Galakrond's Awakening. Reborn minions will revive with 1 Health after they die for the first time.
Similar to Stealth or Divine Shield, Reborn is part of the minion's textbox as well as an ability, meaning a Reborn minion summoned will always be Reborn, and when the effect is "consumed", the textbox will still be active and Reborn can be applied to it again through external means. Reborn minions will not keep any additional enchantments applied to them when they revive, but they can be healed back to full Health as they are considered "damaged" at 1 Health when they revive (except in the case of Murmy which naturally has 1 Health).
In fitting with the desert theme of Saviors of Uldum, all minions from Saviors of Uldum that use Reborn depict mummy-like creatures that are wrapped in bandages.
Keywords that Work Great with Reborn
Reborn minions often have keywords that are beneficial to them anyway. Although they don't keep enchantments after they revive, any additional enchantment applied by you will improve them.
Embalming Ritual or Psychopomp will allow you to apply the Reborn effect to any minion, allowing additional possibilities as to how to use Reborn. Additionally, artificial Deathrattles can be useful to provide a bonus effect after they die. Casting Ancestral Spirit on a Reborn minion for example will summon a damaged 1-Health copy without Reborn as usual, in addition to a fresh new copy with Reborn.
Playing Against Reborn in Hearthstone
Silence a Reborn minion to take out its Reborn effect if you feel the means is justified. Additionally, a small minion that can attack this turn will allow you to take out the revived copy easily and efficiently.
Example Hearthstone Reborn Cards
ShobotBADCARDNAME is a simple 2 mana 2/2 with Reborn and no other ability, effectively a reverse of, and a reference to Shielded Minibot.
Khartut Defender is a Reborn minion with Taunt and a Deathrattle that heals you. On its own, it's a fine card, although it can become extremely powerful (and potentially annoying if played against you) by reviving it multiple times.
Embalming Ritual will allow you to give any minion Reborn, allowing you to place it on a high-priority target or minion with a strong Deathrattle and get the most out of the ability.
Making Mummies is a Quest that requires you to play (not summon) 5 Reborn minions, and will grant you Emperor Wraps upon completing it, synergizing with the Reborn effect as you can copy a minion with Reborn and the 2/2 copy will have Reborn and summon a full-size copy when it dies.