A coven of sea hags: 1,350 xp = 2,700 adjusted, an easy encounter and ~9% of the total XP budget

That's 3 potential reactions to counterspell the party and 3 potential lightning bolts, up to 24d6 or 84 average damage on 3 failed saves or 42 average damage on 3 successful saves to at least 1 party member.

If anyone starts within 30 ft of the hag, they could potentially become frightened at the beginning of the fight. If anyone is frightened, a hag could try to Death Glare it to 0 hit points. However, this is an easy save to make as a DC 11 wisdom save.

They also have option of casting hold person at a higher spell level to catch multiple PCs if they happen to be within 30 ft of each other on later rounds.

This encounter could easily be overcome if 1 hag is killed immediately, breaking the coven. Due to the cheapness of the XP and the yolo nature of their potentially massive damage and/or death glare, I think it could be worthy of inclusion.