If you control "Tristan, Knight of the Underworld", you can Special Summon this card (from your hand). Cannot be Special Summoned by other ways. You can target up to 2 Zombie-Type monsters you control and declare a Level from 5 to 8; they become that Level until the end of this turn, also you cannot Special Summon monsters for the rest of this turn, except Zombie-Type monsters. You can only use this effect of "Isolde, Belle of the Underworld" once per turn.
