If you used Naga, you could always use such gems as Lamia and Ganga. Lamia is one of my more kindred spirits on the Persona series, and Ganga would just fit the theme
... probably some more spirits from the Priestess, and possibly empress arcana as well, as there are some fishy-themed personas in there iirc.
New World Unit Samael
(buildable by Shieam, Kuriotates, and Hippus(if following Judecca) (and Orks?) and perhaps Infernals, as well as maybe Sidar, and anyone who likes to use Necromancy, or who likes dragons ... with the intent that modders will use him for their own pet civ ^_^)
In addition, it would be AWESOME if someone could create a Samael unit with Destroy Undead (Hamaon), Wither (Mudoon), Tsunami (Mabufudyne), Fireball/(Meteor?) (Agidyne), Maelstrom (Mazio/Magaru), Ring of Fire (Maragion), and Stoneskin (Tetrakarn)
perhaps could have dragon race, as well as immune to banish, and +20% vs Angels (name roughly means poison of god) ....
Could have 10 str, hero, +2 (or 4?) death damage, available at Way of the Wicked + Arcane Lore (+ Warhorses?) immune to death damage, possibly immune to unholy damage. Slight weakness to holy damage, perhaps. He is kind of dragon, kind of fallen angel, so I am bouncing between dragon race and demon race.
World Unit, although can buy the immortality promotion at a city with a (demon altar?) (pagan temple?) for 500 gold.
If possible, I would like for each additional level he acquires to increase the cost of immortality by 10 gold.