No, its not from an Un-set or Secret Lair, its from Ikoria: Lair of Behemoths
Wild Mongrel is already a great creature, but when paired with double-strike effects like Double Cleave or Soul Fire, or madness cards like Basking Rootwalla or Reckless Wurm, it becomes one of the best cards in your deck
While this does have a good hit rate it is double black making it that much less appealing to include in most lists
Once you have a reliable way of doing do, Walking Ballista serves as both an excellent creature removal tool and a way to burn your opponents out of the game entirely
Im seriously excited about this card and will undoubtedly be building a deck around it
Casting a spell that targets Brigone gives her a +1/+1 counter on top of whatever bonuses your spell would also give, helping her survive later into a match