Merfolk utilizes lords, pressure, and disruption to win games
Wouldnt cast Commune or burn spells, making it a tougher fit if were not using it for the third color
D&D campaigns are (very often) all about powerful characters that take on evil doers to save the world, or at the very least a town or city, from destruction
Mana Leak is at least the first card so far on this last to be played in the capacity you would expect counterspells to be played in when you need cheap reactive coverall security
But there are still very affordable versions (worth two to three quid) available thanks to The List, a reprint in Battlebond, and Modern Masters
Its really more like two archetypes, forming either end of a scale