I'm not certain about the challenge ratings. I'm going to do some more comparisons to the Monster Manual to determine. It's hard to tell how much tougher the damage resistance makes these creatures.
I tried to make the spell selection as close to the d20 version, but I think I need to add in a "Rage" equivalent because that would appear to be an important component of the tactics of the 3.x version. If you have any suggestions for the spell abilities, let me know in the comments.
Note: This version of the Barghest comes from the d20 System Reference Document with the stat block converted by the D&D 1E/3E to 5E Monster Converter by +Brent Newhall. It is presented under the terms of the OPEN GAME LICENSE Version 1.0a.
A barghest is a lupine fiend that can take the shape of a wolf or a goblin. In its natural form, it resembles a goblin-wolf hybrid with terrible jaws and sharp claws. As whelps, brghests are nearly indistinguishable from wolves, except for their size and claws. As they grow larger and stronger, their skin darkens to bluish-red and eventually becomes blue altogether. A full-grown barghest is about 6 feet long and weighs 180 pounds. A barghest’s eyes glow orange when the creature becomes excited.
A barghest’s natural weapons, as well as any weapons it wields, are treated as evil-aligned and lawful-aligned for the purpose of overcoming resistance. Its natural weapons are treated as magic weapons for the purpose of overcoming resistance.
Occasionally, a greater barghest uses a magic two-handed weapon in combat instead of its claws, giving it two attacks with that weapon. It can also make its bite attack each round.
Medium Outsider, lawful evil
Damage Resistance. Bludgeoning, piercing and slashing from non-magical weapons.
Keen Smell. Advantage on Perception checks related to scent.
Shapechanger. The barghest can use its action to polymorph into goblin, wolf or back into its true form. Its statistics are the same in each form. Any equipment it is wearing or carrying isn’t transformed. In goblin form, a barghest cannot use its natural weapons but can wield weapons and wear armor. In wolf form, a barghest loses its claw attacks but retains its bite attack. It reverts to its true form if it dies.
Innate Spellcasting. The barghest's innate spellcasting ability is Charisma (spell save DC 14)
At will (action) — Blink, Levitate, [Rage - need 5e equivalent]
At will (bonus action) — Pass without Trace
1/day — Bane, Charm Monster , Dimension Door, Nystul's Magic Aura
A barghest advances in Hit Dice by consuming corpses in this fashion. For every three suitable corpses a barghest devours, it gains 1 Hit Die, and its Strength, Constitution, and natural armor increase by +1. Its proficiency bonus and saves improve as normal for an outsider of its Hit Dice.
The barghest only advances by consuming the corpses of creatures whose levels are equal to or greater than its own current total. A barghest that reaches 9 Hit Dice through feeding immediately becomes a greater barghest upon completion of the act. A greater barghest can reach a maximum of 18 Hit Dice through feeding.