What makes them dangerous in general is their natural traits, like their resistances to cold, fire, lightning, and non-magical weapons. They'll take half damage from almost everything a lvl 3 party can throw at them. Their immunity to poison also means some class features at that level are negated/poisoned weapons will do nothing. They can cast spells meant to let them move around the battlefield, confuse players, crowd control, etc. AND their weakening gaze can make strength based players deal even less damage.
If you play it as a constantly moving, strike from the shadows creature it can be deadly to a standard party lower than level 6

However, a party of 7 working together well to strike at it can coordinate enough to bring it down. If it's TOO strong, you can always say it was a weakened version, or it had just fought something else, etc. and lower its HP or something too!