When an opponent's monster declares an attack: You can send this card from your hand to the Graveyard; change the attacking monster to Defense Position. When you Ritual Summon a monster, you can banish this card from your Graveyard as 1 of the monsters required for the Ritual Summon.
One of the staple cards that works very well in any kinds of decks in current card pool due to capability of playing two roles: defending your monsters from being attacked and being a material for ritual summon when it is in the graveyard.
Can be used from your hand and it cannot be negated by most spell/trap cards.
I don't think is a good idea to spend lots and lots of gems only for a defense card. Why should you do that when with that gems you can build a deck for example a deck of a minibox like sylvans,gustos or other f2p decks like aliens?