First I would recommend a "Werewolf, Greater" before trying to get from CR 3 to "Legendary" which, as best I can determine from the MM, means a necessity of CR 13 or better.
So I'd start with a Greater Werewolf.
1) Double the CR (so, 6. Sets them, nicely, 1 above a werebear. Adds to the lycanthrope range and give the werebear something they can actually fear).
2) Makes their HP 12d8 + 36 or, for the average they use in the MM: (1/2 of 96 + 36) 84.
3) Increase Strength to 17 (+3), Con. to 16 (+3).
4) Adjust Claw and Bite attacks to +6 to hit (+3 damage). Give them a different weapon for their humanoid form, if you want.
5) Add damage resistance: poison, necrotic.
6) Add condition resistance to poisoned, charmed (helps add that threat to vampires thing in, if you want that. They're difficult to charm.)
7) Add condition immunity to frightened (other than from a superior werewolf).
8) Alter
Shapechanger: altering form requires only a bonus action. The form is that of a Dire Wolf instead of a normal wolf.
9) Add
Spider Climb. (hybrid form only) Not the best name, but that's what they're using in the MM for something that can climb walls and ceilings. Their claws and preternatural strength should be good for climbing vertical surfaces and upside down...maybe limited upside down movement to half and/or require the use of bother hands and feet. Besides, vamp's have it and if I'm to understand properly, these guys are going to have to be going up against them (and liches and dragons, no less) regularly. So seems like a wise addition.
10) Add
Pack Tactics: as the standard wolf/hobgoblin ability but applies in wolf, hybrid or humanoid forms.
11) Add
Lunar Regeneration, same as a standard troll (10 HP at start of each turn, requiring the fire or acid to stop), with the exception that for three days prior to, during, and 3 days following a New Moon, the werewolf does not/can not regenerate. This is likely one of the reasons there seem to be very few werewolf attacks/sightings when the moon is dark. Regeneration is also stunted by silver damage, see
Silver Vulnerability below.
...annnnd...
12) Add
Silver Vulnerability. The werewolf can not abide the touch of silver. Objects, whether weapons attacks or not, made of silver that touch the greater werewolf deal 6 (1d8 +2) of radiant damage to the werewolf. This damage continues each round they remain in contact. Damage taken by silver comes off of the werewolf's hit point maximum and requires a full 24 hours [per 6 HP, or
per long rest if you prefer the terminology] to regenerate/restore the werewolf's hit point maximum to normal. Additionally, they will not cross even trace amounts of the harmful metal, scrolling in doors or walls, built into doorframes/thresholds, lines of powdered silver dusting window sills or other portals, etc... though will always seek to circumvent it if they can/have some reason to (otherwise they will simply seek out easier/safer hunting elsewhere).
There's a "Greater Werewolf/Lycan/Lupomorph/whatever you want to call them."
From there, it is not so far a jump to
"Legendary Werewolf"/"Pack Source"/"Alpha Elder" or...if I may, the
"Worglord", or whatever you want to call them.
In addition to all of the traits of the Greater Werewolf, the Worglord receives the following attributes:
1) Increase to CR 13.
2) HP: 19d8 +72 (148)
3) Strength to 20 (+5), Con. to 19 (+4), Claw/Bite/Rend (see below) attacks +10 to hit, +5 damage.
4) Damage resistances: Necrotic, Silvered non-magic weapons.
5) Damage Immunities: Non-magic weapons that aren't silvered, poison
6) Condition Resistances: Prone, Grappled, Stunned
7) Condition Immunities: Charmed, Frightened, Restrained (other than by silver).
Now, the good stuff...
8)
Shapechanger: altering form requires only a bonus action. The form is that of a worg (or dire wolf, if you don't want to go worg) of the largest size instead of a wolf and the Worglord can speak normally in any language they know while in all forms.
9)
Superior Hearing and Smell: is +5 and has advantage on Perception checks.
10)
Darkvision: to 120'
11)
Legendary Resistance: as normal.
12)
Regeneration: as the vampire (20HP at start of each turn) except sunlight, running water, and holy water have no effect on the Worglord. If they take damage from radiant, fire, acid, or silver damage, they are prohibited from regenerating the following round. But after that, even damage from silver regenerates normally. The Worglord is also capable of regenerating regardless of the phases of the moon.
13)
Silver Vulnerability: as above, except the Worglord can regenerate silver damage normally after 1 round of having their regeneration stunted, as detailed under
Regeneration.
Actions:
11)
Multiattack: (any form) The Worglord makes
3 attacks, 2 with its claws and 1 with its bite.
12)
Shredding Claws:(Humanoid or Hybrid forms only) Melee Weapon Attack The Worglord can partial shape its humanoid hands into its werewolf claws. With these claws it can make two separate claw attacks against two different targets within its reach.
13)
Spider Climb, as above.
14)
Rend: Melee Weapon Attack If the Worglord hits with a claw attack, it can take a bonus action to attack with the other even if they've already made a second claw attack. If the second claw hits, the Worglord
rends the target, claw damage from each attack is automatically maximum (including Strength bonus) and doubled.
15)
Lord of Lupines: Werewolves (greater or lesser) and wolf-based creatures (including worgs and winter wolves) will not willfully attack the Worglord. Any creature being otherwise controlled or commanded to do so must beat a DC 15 Cha. check to attack at all and, if successful, their attack is made with disadvantage. Wolves and lupine creatures not otherwise being controlled will follow verbal commands of the Worglord. If under some other charm or magical control, the Worglord can use an action to make a Cha. check against the DC of the magic controlling the wolf creatures to usurp control of them. This usurps control of all creatures effected by the same magic/effect regardless of total CR or numbers.
16)
Howl of Horror (recharge 4-6): The Worglord uses one of the following Howl attacks.
- Horrifying Howl: All non-wolves/werewolves within 300' must make a DC 17 Wisdom save of suffer the Frightened Condition for one minute.
- Stunning Howl: The Worglord pitches a howl into a 30' cone of focused soundwaves. All creatures within the area must succeed a DC 13 Constitution check. On a failed save, the creature is Deafened for 1 minute and Stunned until the end of their next turn. On a successful save, the creature is deafened but not stunned.
- Summoning Howl: The Worglord sends out a howl to its minions. Up to 3d8 wolves or 2d8 dire wolves (or winter wolves if appropriate to the terrain) or 2d6 werewolves. They arrive in d4 rounds, will fight to the death and otherwise follow all commands of the Worglord.
Legendary Actions: The Worglord gets 2 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The worglord regains legendary actions at the start of its turn.
- Move: the Worglord moves up to speed without provoking opportunity attacks.
- Shapechange: The Worglord can alter their form from Humanoid to Hybrid to Wolf/Worg-form or any variation thereof.
- Multiattack: the worlgord takes its normal 3 physical attacks.
- Howl: the worglord can make a roll to see if its howl ability recharges. If it does, they may use this legendary action to make a howl attack if they wish.
