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.
I don't get it why if it is flipped face down with Canadia when you flip it up again it loses all it effects gained from the ritual materials (like Revenants). Also If you summon it with Evolution it is supposed to be destroyed at the end of a turn, but if flipped down it isn't Can someone explain?
When flipped face-down, it basically forgets anything that were applied to it back when it is face-up, because a face-down card essentially has no identity. It is treated as if no one knows what is it.