A Ritual Summon (儀ぎ式しき召しょう喚かん Gishiki Shōkan) is the act of Special Summoning a Ritual Monster that is conducted through the effect of a Ritual Spell Card. These Summons usually require Tributes greater than or equal to the Level of the monster being Summoned from your side of the field or your hand. These Tributes are treated as a Card Effect rather than a Cost. However, some cards, such as "End of the World" require them to have the exact Level requirement. With "Advanced Ritual Art" you can send Monsters from your Deck to the Graveyard instead of your hand or Field. "Gishki Photomirror" does not require Monsters to be Tributed, it requires 500 Life Points x the Level of the Ritual Monster.
A Monster that was properly Ritual Summoned can be Special Summoned from the Graveyard unless otherwise dictated by the card's text.
How to Summon a Ritual Monster sửa During your Main Phase 1 or 2, if you have a Ritual Monster in your hand, its required Ritual Spell Card in your hand or Set on the field, and a monster(s) on the field or your hand whose total Level(s) meet the requirements of the Ritual Spell Card, you can activate the Ritual Spell Card.
Reveal the Ritual Monster in your hand that you are attempting to Ritual Summon. Tribute monsters from a location(s) specified on the Ritual Spell Card, whose total Levels meet the requirements of the Ritual Spell Card. (Unless the Ritual Spell Card specifies otherwise, you are allowed to Tribute face-down monsters you control.) (If all your Monster Card Zones are currently used, you must Tribute at least 1 monster in them to Ritual Summon the Ritual Monster.) Special Summon the Ritual Monster Card onto your side of the field in either face-up Attack or Defense Position. Finally, send the Ritual Spell Card to the Graveyard upon resolution of its effect.