Okay, maybe call this attack Icy Clutch then (or Death(ly) Clutch)? - sounds like a good mechanic.
I was thinking
Doom Grip to match the
Doom Strike.
Putting all the notions I had together looks like this:
Doom Grip. Melee or Ranged Weapon Attack: +8 to hit (automatically hits a restrained opponent), reach 15 ft. or range ##/###, one target. Hit: ## (#d# + #) psychic damage plus ## (#d# + #) necrotic damage and the target is grappled (escape DC ##). Until this grapple ends, the target is restrained, and the cairn shadowraith can't use Doom Grip or Doom Strike on another target. The target must succeed on a DC ## Constitution saving throw or its hit point maximum is reduced by an amount equal to the necrotic damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.
In addition, if the target is a creature, it suffers 1 level of exhaustion if it fails the saving throw. If the target fails the saving throw by 5 points or more, it suffers an additional level of exhaustion. This exhaustion remains until it is healed magically: a lesser restoration can remove one level of exhaustion from a victim with two or fewer levels, a greater restoration will remove three levels of exhaustion. The target dies if it suffers 6 levels or exhaustion.
Yes might nudge the CR up but that's ok
Okay, plugging the numbers of the 5E Wraith into the CR Calculator says it is Challenge Rating 4 but the actual undead is
Challenge 5, suggesting the authors reckon its wraithly powers (i.e. condition immunities and
Life Drain) are worth another +1 CR or possibly +20%. I'm still unclear as to how special abilities are supposed to effect CR.
The current Shadowraith has a Defensive CR 13, which implies that if we set the damage to a range that gives it Offensive CR 13 (which is DPR from 75 to 85) to make it an "unadjusted" Challenge 13 then its actual
Challenge would be 14 or possibly 15, depending on how nasty we reckon it's various life sapping powers are.
Alternatively, we could lower the DPR to make its unadjusted Challenge 12, requiring an Offensive CR or 10 (DPR 63 to 68) or 11 (DPR 69 to 74), or possibly even Challenge 11 with Offensive CR 8 (DPR 45 to 56) or 9 or (DPR 57 to 62) and guesstimate the final creature as
Challenge 13.
Hmm… I favor the second approach and keep it
Challenge 13 with a DPR of, say, 60 or thereabouts. That way the cairn shadowraith would have basic attacks doing similar damage to the cairn wight's basic attacks (i.e.
Skull Mace), which seems appropriate as they're closely related.
The special attacks would do a bit less damage. They're comparable to the cairn wight's
Soul Drain, which does roughly two-thirds the damage of its
Skull Mace.
Do those numbers sound reasonable to you?