Woah this looks slick! I started writing this as a comment but it quickly grew into something warranting a discussion, so I'm starting this here.

Like others here: I would love OvN compatability (a must-have for me). I have some suggestions on both making that possible and balance:


The Fimir start with 3 settlements: Baersonling's Camp, Frozen Landing, and Floating ____ which is way in the south, where their starting Lord also is. The first two of these are competing for Mortkin's starting settlements.
Fortunately, I don't think the Fimir need 3 starting settlements, so we can give one to Mortkin. Mortkin can make do with only starting with 1, especially given how laughably overpowered he is (more on that later). However an important OBJECTIVE is to not have the Fimir get curb-stomped by Mortkin in AI vs AI every single game. The Fimir should have a possible presence in the north, otherwise there's no point for them to exist.
So, which settlement to give?
In OvN, Frozen Landing is a lvl 1 settlement and Baersonling's Camp is lvl 2. Fimir's Lord also spawns in the south, so the north starts pretty undefended. Fimir would be shafted in the north if they were left only with a lvl 1 Frozen Landing.
On the other hand, I think it would be more flavorful for Mortkin to have Baersonling's Camp, which also flows better for your initial war on Kislev/Empire that you have set up at start.
So, I give this suggestion this from the perspective of someone that hasn't tried modding WH2 (I mod Starcraft, though): perhaps an OvN submod is in order?
This way if OvN isn't used, this mod can continue as-is. And if OvN is in play, the submod can kick in.
Via this submod the Fimir's Frozen Landing could be buffed to lvl 2. Additionally it could incorporate the Mod Configuration Tool] for the player to choose whether to spawn the Fimir's starting army in the south or the north.

BUG: Mortkin's Monster Hunt technologies are available for the Fimir to also research even if this mod is turned off.


In my limited testing so far, this mod IS compatable with Kislev Reborn if Kislev Reborn has a higher position in the load order. However Kislev Reborn creates two new factions that comprised the vanilla lands of Kislev. If it's your intention to start at war with these factions, then a quick submod to do so should be able to wrap that up and also extend the Black Iron Reavers Leadership faction ability to the two new factions.


Alright, to be frank: Mortkin is the most overpowered custom Legendary Lord/Faction I've ever played with.
I have several suggestions to help improve the balance (while still keeping him incredibly powerful). A few of these are buffs, but they're islands in a sea of nerfs.

BUG:
units recruited by Mortkin's horde are lvl 8. I don't see what's causing this. By what I see they should be lvl 4 (I recommend nerfing this further anyway).

TRAITS & START:
-reduce Black Iron Reavers faction trait's Chaos Corruption from +2 to +1
-reduce Black Iron Reavers faction trait's upkeep reduction for Marauders from -15% to -10%
-reduce Black Iron Reavers faction trait's leadership bonus from fighting Empire/Kislev from +15 to +10
-reduce Lord of the Fell Legion trait's Physical Resistance from 10% to 8%
-reduce Lord of the Fell Legion trait's Recruit Rank from +4 to +2
-change starting army composition: Lord Mortkin, Crimson Reapers, 1 Marauders @ lvl 2 (since a newly recruited Marauder would be lvl 2), 1 Marauder Champions (Great Weapons) @ lvl 0, 1 Marauder Berserkers @ lvl 0. (Yes, only 5 units. You can still locally recruit 5 a turn lol it'll be fine.)

BUILDINGS:
-reduce horde tier 1 Black-Iron Fastness's Horde Recruitment Capacity from +2 to +1
-reduce horde tier 1 Black-Iron Fastness's Income Generated from 200 to 100
-reduce horde tier 2 Black-Iron Stronghold's Income from 300 to 225
-reduce horde tier 3 Black-Iron Hall's Income Generated from 400 to 375
-increase horde tier 4 Black-Iron Fortress' Income Generated from 500 to 550
-increase horde tier 5 Alsvartsborg's Income Generated from 600 to 750
-move Marauders (Great Weapons) recruitment away from horde tier 1 Marauder's Gathering Place and move it to horde tier 2 Warrior's Drinking Hole. This is still a buff from the normal settlement recruiting chain as it doesn't require a Smithy.
-move Marauder Champions recruitment away from horde tier 2 Warrior's Drinking Hole and move it to horde tier 3 Champion's Meadhall.
-reduce horde tier 1 Marauder's Gathering Place "-10% upkeep of Marauder units" to "-2% upkeep of Marauder units"
-reduce horde tier 2 Warrior's Drinking Hole "-20% upkeep of Marauder units" to "-3% upkeep of Marauder units"
-reduce horde tier 3 Champion's Meadhall "-30% upkeep of Marauder units" to "-5% upkeep of Marauder units"
-reduce horde tier 3 Champion's Meadhall casualty replenishment rate from +15% to +4%
-reduce horde tier 4 Champion's Proving Grounds "-10% upkeep of Black Iron Reavers" to "-5% upkeep of Black Iron Reavers"
-reduce horde tier 5 Champions' Blood-Field "-20% upkeep of Black Iron Reavers" to "-10% upkeep of Black Iron Reavers"
-reduce horde tier 5 Champions' Blood-Field "+10 Melee attack for Black Iron Reavers" to "+4 Melee attack for Black Iron Reavers"
-reduce horde tier 5 Champions' Blood-Field "+10 Melee Defense for Black Iron Reavers" to "+4 Melee Defense for Black Iron Reavers"
-move Skin Wolves recruitment away from horde tier 2 Ice-Wolf Den to horde tier 3 Skin-Wolf Clan
-reduce horde tier 3 Norse Runemaster recruit rank from +4 to +3
-increase horde tier 3 Black Iron Hall population cost from 3 to 4
-increase horde tier 4 Black Iron Fortress population cost from 4 to 6
-increase horde tier 5 Alsvartrborg population cost from 5 to 8
-reduce horde tier 1 Warhound Kennels population cost from 4 to 2
-reduce horde tier 1 Raider's Sheiling population cost from 4 to 2
-reduce horde tier 1 Sacrifical Alters population cost from 5 to 3
-reduce horde tier 1 Prisoner Pens population cost from 4 to 2
-reduce horde tier 1 Norse-Flesh Brander population cost from 4 to 2
-increase horde tier 5 Neiglensve, Shornaalsve, Tcharsve, and Kharnathsve buildings population cost from 5 to 10
-increase horde tier 4 Altar of Skulex population cost from 4 to 5
-reduce horde tier 5 God-blessed Altar of Skulex population cost from 4 to 1

MORTKIN STATS:
-increase Mortkin upkeep from 250 to 300
-reduce health from 4506 to 3925
-reduce leadership from 110 to 70
-reduce speed from 38 to 31
-reduce melee attack from 65 to 60
-reduce melee defense from 66 to 64
-reduce charge bonus from 64 to 44
-increase base weapon damage from 100 to 225
-reduce armor-piercing damage from 425 to 300 (total 525)
-remove bonus vs large
-increase bonus vs infantry to 25
-set Ward Save, Physical Resistance, Magic Resistance, and Fire Resistance to 0
-set Missile Resistance to 15
-set Splash Target Size to Large
-set Splash Max Attacks to 8
-set Attack Interval to 4.7
-remove Skaven Plague resilience

MORTKIN SKILLS:
-change blue skill line "High King" to +3 Horde Growth, +6, and +9 for the three levels respectively
-change Warrior Born's Melee Attack to +4, +8, and +10
-change Blade Shield's Melee Defense to +4, +8, and +10. Reduce the physical resistance on lvl from +8% to +5%
-reduce Dark Plate's lvl Armour from +6 to +5
-reduce Unholy Resilience's health points to +4%, +9%, +15%
-reduce Northern Fury's Weapon Strength to +4%, +8%, and +10%
-change Vengeance of the Fell Legion's passive ability range from Map-Wide to 40m
-reduce Vengeance of the Fell Legion's passive ability Melee Attack from +16 to +8
-add Disabled If condition to Vengeance of the Fell Legion. Disabled if Leadership <50%
-reduce all Weapon Strength bonus from Everchosen's Favor skill. From +25% to +0%
-reduce Everchosen's Favour ability duration from 45 to 30
-eliminate Everchosen's Favour ability cooldown and assign 1 use charge. Make a once-per-battle ability.
-reduce Everchosen's Favour ability splash attack power from +100% to +50%
-reduce Everchosen's Favour ability base weapon damage from +50% to +35%
-increase unlock Black Axe of Mortkin ability from lvl 8 to 10
-increase unlock Black Plate of Mortkin ability from lvl 9 to lvl 12
-reduce Black Axe of Mortkin unique equipment's melee attack from +15 to +12
-reduce Black Axe of Mortkin unique equipment's weapon strength from +20% to +15%
-reduce The Black Axe of Mortkin ability's Armour-Piercing Damage from +75% to +25%
-reduce The Black Axe of Mortkin ability's splash attack power from +50% to +25%
-reduce Blackened Plate of Mortkin unique equipment's Ward Save from +15% to +6%

UNITS:
-increase Black Iron Reavers upkeep by 40%
-reduce Black Iron Reavers melee attack from 52 to 50
-reduce Black Iron Reavers melee defense from 76 to 74
-increase Black Iron Reavers (Great Weapons) upkeep by 40%
-reduce Black Iron Reavers (Great Weapons) melee attack from 58 to 56
-reduce Black Iron Reavers (Great Weapons) melee defense from 60 to 58
I don't know what their behind-the-scenes stats (attack interval, etc.), are. Can you share?
Also all the stats of the Fire Dragon too, please.

With those balance changes, our boi Mortkin should still easily be the baddest man around town, which seemed to be what you were going for. Just not as overwhelmingly so. ;)

Thanks again for making the mod! I'm looking forward to really diving into it when it's closer to the finished state!