The Reborn system is a special type of Evolution where a high Tier monster (some of which are extremely hard, or even impossible to farm) can be made using Hybrid Monsters or two 4-StarTier 1 monsters as materials.
Rebirthing a monster also costs 280,000 Gold (modified by Castle racial evolve bonuses).
Note: If both of the material monsters are Elite the result monster will be Elite as well (gaining a +30% bonus). Other bonuses are NOT carried over.
Tier 5 units require two Hybrids which can be found through Raids and and three more which are Reborn as the product of two other Hybrids. ( t5 reborn is paused, in other hands, it not available at this moment)