Wight

A wight on the party's side is a hard sell, and the sell is the encounter. The most playable framing is a wight bound by a relic the party carries, a wight whose unfinished business aligns with the party's quest (avenging the death that made them undead, returning a stolen object), or a wight serving as a guide through a sealed undead-only passage. Make the alliance time-limited and contractual. The wight will help with one task, and after that the party should expect to fight it.

In combat the wight does roughly 20 average damage per round through Multiattack and adds Life Drain on demand for max-HP reduction on enemies the party wants permanently weakened. Aim it at named enemies, not mooks. The HP-max debuff is wasted on a goblin and devastating on a recurring villain. The Necrotic Bow at +4 to hit, range 150/600 ft., for 10 average damage means the wight can hold the back rank and snipe while the party closes.

The Sunlight Sensitivity is your scheduling tool. The wight only operates well at night or underground, which limits when the party can use it and forces interesting decisions about timing. The party who plans a dawn raid loses their undead ally for that fight. The party who plans a midnight infiltration has it.

Decide before the session what the wight wants in return, and let the party hear part of it. A name, a key, a body. The cost should be uncomfortable but not unthinkable, and the players should debate whether to pay.

A wight is the undead lieutenant the party meets before whoever owns the crypt. Run it as a ranger officer who happens to be dead. It uses both a Necrotic Sword and a Necrotic Bow, which means it switches between melee and 150 ft. range without losing tempo, and it commands the zombies it has raised.

Multiattack is two swings combining the sword and the bow in any combination, and one of the swings can be replaced with Life Drain. Life Drain is the centerpiece. DC 13 Con save within 5 ft., 6 necrotic on a fail, and the target's HP maximum drops by the damage taken. That is a permanent (until a long rest with appropriate magic) hit point loss, and the players notice. A PC walking out of the fight at 12 max HP instead of 30 is the kind of consequence that makes the next encounter scarier without the GM saying a word. Open with Life Drain on the cleric. Their healing comes off a smaller pool now, and the rest of the fight bends around that.

The kicker on Life Drain is the zombie clause. A Humanoid killed by Life Drain rises 24 hours later as a Zombie under the wight's control, up to twelve. If the party loses a PC to the wight and does not destroy or restore the body, that PC walks back into the campaign next session as an enemy. Telegraph this. The wight should be standing in a room full of obviously-recent corpses that match the local townsfolk the party met two sessions ago.

Sunlight Sensitivity is the structural weakness. Disadvantage on attacks and ability checks in sunlight means the wight will not fight outdoors during the day if it has any choice. Players who figure this out and force the encounter into noon get the bonus they earned.

Have the wight speak. It remembers being alive, badly. One line, delivered before the fight starts, in a voice that almost sounds like the person it used to be. The party should hesitate before swinging.