Darkstone Sage

[the missing waifu]

Darkstone Sage
Effect Monster
Level 1 DARK-Rock
100 / 100
Once per turn, when your opponent sends a card frrom their side of the field to the Graveyard: You can Special Summon this card (from your hand), and if you do, banish that card, face-down. If this effect is negated, then you can target 1 monster on the field: That monster cannot leave the field, except from battle.

[Caláin]

Hmm. On the one hand, the last effect would make something like Marshmallon virtually invincible. On the other hand, the effect is propably hard to activate – if you control a monster that cannot be destroyed by battle, your opponent will hardly allow it to become completely invulnerable just to save one card from being banished from their graveyard. In the end, it looks to me as if you had the best chances if you negate the effect yourself (if you find a card which can do that, for I do not know one). Anyhow, there propably is some room for interesting combos (but with the possibility to create an invincible monster, I'd do something about that if I were you). Apart from that, I'm not sure if I understood the first sentence correctly (whether "his/her card" means "a card they control", "a card in their hand", "a card from their deck", all of these or something completely different).

[the missing waifu]

Sorry, I mean on the field.