You can Ritual Summon this card with any "Vendread" Ritual Spell. Once per battle, if this card battles an opponent's monster, during damage calculation (Quick Effect): You can banish 1 Zombie monster from your GY; this card gains 300 ATK. If this Ritual Summoned card is sent to the GY: You can add 1 Ritual Spell from your Deck to your hand, and if you do, send 1 "Vendread" monster from your Deck to the GY. You can only use this effect of "Revendread Slayer" once per turn.
Attack boost is small and requires you to banish a Vendread monster in - your graveyard.
When sent to the graveyard, Vendread Slayer’s search effect lets him add a Vendread Ritual Spell to the hand and pitch a Vendread monster to the graveyard. This is especially good in a Vendread deck because their Ritual Spells can summon from the graveyard.
Use these cards to retrieve your banished Vendread monsters. Vendread Revolution can be banished from your graveyard to retrieve 5 banished Zombie monsters and draw a card. Vendread Reunion is essentially lets you Ritual Summon with banished monsters as material, returning those banished monsters to the graveyard.