There's also a bit at the end of the Steam DLC page as well as the F.A.Q. page: "But this is not all. The Druchii possess a dark and powerful weapon.
When blood runs in rivers and the air is choked with Death, the chains will break and this monstrous secret will be unleashed."
People are guessing this is another unit that's not been shown yet, being a toss up between the Kharibdyss or the Blood-wrack Medusa. The first would seem more likely since it's similar to a Hydra however the Medusa is more thematically appropriate.
Some of the regiments of renown seem quite neat. There's a unit of Phoenix Guard who burst into fire whenever one of them dies, a unit of Dark Riders mounted on dark pegasi, a war hydra with frostbite and cold breath and a unit of shadow warriors that can fire while remaining hidden as well as an ability "Loec's Shroud" that increases their speed and reduces detection radius.
Regiments of Renown:
Avelorn (Alaraielle)
The Scions of Mathlann (Spearmen)
‘Aura of Protection’ passive ability (applies a ward save to nearby units whilst mana > 50%)
Expert Charge Defence
The Puremane Company (White Lions of Chrace)
‘Guardian’ passive ability
‘Sunder Armour’ contact effect
Keepers of the Flame (Phoenix Guard)
Magic Attacks
Mark of Asuryan (Entities will burst in to flames upon death and deal damage to any nearby enemies i.e. New interpretation of the Dwarf Slayer ‘Death Blow’ trait)
The Storm Riders (Lothern Sea Guard)
Cause Fear
Vanguard Deployment
Fire Whilst Moving
The Grey (Shadow Warriors)
‘Loec’s Shroud’ ability (Increases speed and reduces the detection range significantly for enemies to spot them for the duration of the ability)
Snipe attribute (able to fire and remain hidden)
Everqueen’s Court Guard (Sisters of Avelorn)
Encourage
‘Banner of Avelorn’ ability (Increases Winds of Magic recharge rate)
The Fireborn (Dragon Princes)
Bonus vs Large
Fire Attacks
Heralds of the Wind (Ellyrian Reavers – Bows)
Increased speed
Superior melee stats
Har Ganeth (Hellebron)
The Hellebroni (Dreadspears)
Poison Attacks
Expert Charge Defence
Sisters of the Singing Doom (Witch Elves)
Cause Terror
Blades of the Blood Queen (Har Ganeth Executioners)
‘Guardian’ passive ability
‘Frenzy’ passive ability
The Bolt-fiends (Darkshards – Shielded)
‘Shieldbreaker’ contact effect (missile hits from this unit reduce missile block chance of target)
Additional range
Slaanesh’s Harvesters (Doomfire Warlocks)
‘Word of Pain’ bound spell
‘Soul Stealer’ bound spell
Knights of the Ebon Claw (Dread Knights)
Does not Rampage like other Cold One units
Murderous Prowess upgraded to Murderous Mastery
Chill of Sontar (War Hydra)
‘Frostbite’ Contact Effect
‘Fire Breath’ with ‘Frost Breath’ which deals more damage and applies the ‘Frostbite’ contact effect
Raven Heralds (Dark Riders)
Mounted on Dark Pegasi
It's also interesting that Alarielle can recruit units of dryads, treekin and treemen, giving access to some of the Wood Elf units to at least her faction for the high elves.
Both Alarielle and Hellebron also have mechanics during the campaigns which affect their abilities.
Alarielle's strength seems to be tied to the amount of Chaos Corruption within either the world or Ulthuan (it doesn't really say) and also gains bonuses or penalties depending on whether there's enemies within Ulthuan or not.
Hellebron has a mechanic where she loses power unless you use the "Death Night" rite that also summons an indepenent army of Dark Elves you gain the post-battle rewards from.