Once you achieve level 80 in two remort classes, you can gain in one of the six realm lord classes. Each class has its own specific bonus that increases by level and very powerful skills. When the maximum realm level (30) is reached, the character will receive their full bonus.

Living Shadow	Bonus to critical hits and percentage of circles and backstabs that crack.(Deal critical damage)
Blademaster	Physical damage is increased significantly.
Malakim	Hitpoint regeneration is increased significantly.
Deity	Deities get the ability, "Layhands," which restores a percentage of their target's health once per mud day. At full bonus, layhands is a complete restore.
Exorcist	On 50% of damage dealt (except for Damage over Time (DoT) spells), Exorcists will leech up to 30% of the damage they deal back in HPs without any change in alignment.
Dark Templar	Mana regeneration is increased significantly.