Shadowverse

But... it's not very strong on its own, and in the late game, a 6/5 rush with Vengeance will be much more effective. But still, I think it's not enough to save Blood.

Examples like Venus, Ariana, White Wyrm, Tana, Draconimorph, Luminos, and Monk with Holy Water have long shown that weak and mediocre cards can gain a second life when they become tokens for another decent card. In this case, it's the same – you don't waste slots in your deck and don't cripple yourself in the game with your stupid Blood Moons; instead, you play a normal 3-drop that gives you this additional functionality.
For now, it's the best card for including Vengeance in the history of Shadowverse. A standalone Blood Moon, for the reasons mentioned above, is not viable, and Belf is just garbage with an idiotic design. You draw two cards and pay half your health for it.
And be prepared for the Blood player to highroll an Imp-Guy-Blood Moon in attack, and you don't have any low-cost removal in your hand to kill it. Then, they finish you off with an evolved General and a Silver Chain.
I won't predict whether this will "save" Blood, but it's clear that this legendary card is excellent for this deck. And don't forget that for about a month after its release, you can still run Belf and Dirigibles in the same deck to spend the entire game in permanent Vengeance, maximizing the highroll potential without losing health on your hero.