Abhorrent Adversaries of the Afterlife

Completing all the legendary combat pets in Shadowlands will reward Winterleaf Spriggan
There are no dailies or any kind or any other gated mechanics other than you unlocking the zone and defeating the below.
Full disclaimer that I did not write up these strategies these are from Xu-Fu. Also I would highly recommend reading the comments and there are some excellent suggestions of strats there too!

Rivendreth
Sewer Creeper
/way 25.67 23.62
Kill Team:
Bloodfeaster Larva * 2 1
Motorized Croaker 1 * 2
Bloodgazer Hatchling * * 1.
Creeping Ooze x2, Burst, switch to Croaker, Water Ket, explode, switch to hatchling, Predatory Strike

The Countess
/way 53.00 41.50
Kill Team:
Fragment of Anger 1 2 1
Fragment of Desire 2 2 1
Fragment of Suffering 1 * 2
Enrange, Soulrush, Spiritfire bolt until Anger dies. Switch to Desire. Priority 1: Use Soul Ward at the same round The Countess uses Lift-Off. Priority 2: Soulrush Priority 3: Arcane Blast. If things go bad, bring in suffering and dish out pain with soulrush and arcane blast.

Bastion
Crystalsnap
/way 52.71 74.29
Kill Team:
Unborn Valk * 2 1
Ikky * 1 1
Curse of Doom, Haunt, switch to Ikky, Black Claw - Flock

Digallo
/way 26.22 30.46
Kill team:
Pandaren Fire spirit 2 * 1
Magma wave x 4, Cauterize, Magma wave x 5, Cauterize

Kostos
/way 46.53 49.31
Kill team:
Boneshard 1 1 2
Ikky 2 1 1
Blistering Cold, Bonestorm, Chop (Boneshard dies) switch to Ikky, Black Clare - Flock
Edited to include Catsniffer99's comment: Bring a pet with Blistering Cold along with Ikky with Black Claw and Flock for Kostos; it is a faster kill team.

Maldraxxus
Gelatinous
/way 61.90 78.80
Kill team:
Lunar Lantern 1 1 1
Mechanical Pandaren Dragonling * * 1
Darkmoon Zeppelin * 2 *
Beam, Sould Ward, Illuminate, Bring in Mech Pandaren Dragonling, explode, bring back Lunar Lantern, Sould Ward, Beam, Illuminate.
Edited to include Catsniffer99's comment: This fight can be solo'd with an Emerald Proto-Whelp.

Glurp
/way 26.49 26.75
Kill team:
Rabbit 2 2 1
Elfin Rabbit 2 2 2
Brown Rabbit 2 2 1 (any breeds and you can use any bunny really)
Dodge, Flurry, next pet will be hooked in, Flurry, Dodge, Rinse repeat until dead.

Ardenweald
Briarpaw
/way 34.22 44.52
Kill team:
Rabbit 1 2 1 (any breed, lots of bunnies to choose from)
Scratch 2 x, Dodge, Scratch x2 Burrow.

Chittermaw
/way 26.58 62.17
Kill Team:
Baa'l[/url ]* 2 *
Iron Starlette * * 2
Mechanical Pandaren Dragonling * * 1
Murder the innocenet, switch to your IRon starletter, Explode. Switch to mech panaren Dragonling, explode, Switch to Baa'l Murder the Innocent.
- Edited to include Catsniffer99's comment: If you don't have Baa'l for Chittermaw, you can use a pet with Glowing Toxin, a pet with Corpse Explosion, and then a pet with Surge of Power as an alternate strategy. (You can easily search for pets with these abilities with the search feature in the pet journal)
- Edited to include hellvis Comment: If you struggle with these pets or don't have them, you can use
Darkmoon Zeppelin decoy, explode, bring in Chrominius- Howl, Surge of Power x 2 - I have not tested this strat out but if you don't have Baa'l it is worth a shout.

Mistwing
/way 49.88 41.75
Kill team:
Unborn Valk * 2 1,
Elfin Rabbit * 2 2 (>319 speed bunny)
Widget the Departed * * 1
Curse of Doom, Haunt, switch to Elfin Rabbit, Dodge, Stampede until Wabbit dies, switch to widget, Call darkness!

Congratulations on your very own [item=184512]

TomTom:
Bastion
/way #1533 52.71 74.29 Crystalsnap
/way #1533 26.22 30.46 Digallo
/way #1533 46.53 49.31 Kostos

Revendreth
/way #1525 25.67 23.62 Sewer Creeper
/way #1525 53.00 41.50 The Countess

Maldraxxus
/way #1536 61.90 78.80 Gelatinous
/way #1536 26.49 26.75 Glurp

Ardenweald
/way #1565 34.22 44.52 Briarpaw
/way #1565 26.58 62.17 Chittermaw
/way #1565 49.88 41.75 Mistwing

TomTom:
/way #1525 25.67 23.62 Sewer Creeper
/way #1525 53.00 41.50 The Countess
/way #1533 52.71 74.29 Crystalsnap
/way #1533 26.22 30.46 Digallo
/way #1533 46.53 49.31 Kostos

/way #1536 61.90 78.80 Gelatinous
/way #1536 26.49 26.75 Glurp
/way #1565 34.22 44.52 Briarpaw
/way #1565 26.58 62.17 Chittermaw
/way #1565 49.88 41.75 Mistwing