First, we have this post with some datamining done by Ghidoran on the gamefaqs forums:
http://www.gamefaqs.com/boards/genmessage.php?board=944424&topic=49643868
To Quote Ghidoran (his post has more details, this is just the general information):
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In an interview I read that one of the secret characters is in the file structure of Demigod, so I went about searching for clues. I tried the dgdata folder first. In "characters", figured this:
(The ones on the right are the names in the folders, the ones on the left are(what I think) they correspond to)
Angel - Angel
Archer - Archer
Bird - ?
Ent - Shambler
Flag - Flag
Giant - Giant
HighPriest - Summonable Priest
King - Citadel(?)
Leopard Rider - Sedna
Mage - Torchbearer
Minotaur - Soldier
MinotaurCaptain - Summonable Minotaur
Oak - Oak
Priest - Priest
Queen - Queen of Thorns
Rook - Rook
Serpent - Serpent in Leviathan map
SiegeArcher - Summonable Archer
Sniper - Regulus
Spirit - Spirit
Trebuchet - Trebuchet
Unclean_Beat - Unclean Beast
Vampire - Night Walker
Vampire_Lord - Lord Erebus
WarIdol - ?
WhipTail - Catapultasaurii
Yeti - Yeti
So far Bird, King and WarIdol are the only ones I don't know.
In the "units" section there's the "heroes" folder, and there's 9 folders there. One for each Demigod, BUT THERE's ONLY 8 DEMIGODS DUN DUN DUNNN. Okay, so I opened up the skilllayout file, easiest to figure out, and checked em all out. Perfect. Now, most have names, except the first 3. HEMA01 is Torchbearer, HEPA01 is Unclean Beast, and HGSA01 is Regulus. You'll notice there's an extra one, called HOculus. There's an H in front of all of them so it's just Oculus. New Demigod? Perhaps.
Abilities:
Omniscience: The death of Oculus\' foes extends his vision. Every 3 enemies killed increases his sight range by 1. When his sight range reaches its maximum, an Eye is summoned, and Oculus\' sight range reverts.',
Far Sight: 'Oculus\' sight range and weapon range increase by 3.',
Ignus Fatuus: Oculus creates a will-o\'-wisp near the edge of the field every 10 seconds, luring enemy soldiers over the edge and to their death.'
Prism: Oculus refracts the beam from a Tower of Light, deflecting 25% of its damage to all nearby enemies and allies.
Salvage Armor: Decreases the cost to buy units at your castle by 10%
Dark Shroud: Oculus throws his enemies into darkness, reducing their sight range by 20%.
HelpText = 'Strengths:\nReduces vision of enemies\nTakes reduced damage from Towers of Light\nSends weaker units to their death\nCan summon eyes to scout for him\n\nWeaknesses:\nReduced vision\nLow Hit Points\n\nWhile Oculus\' sight is initially weak, it will become more powerful than any other Demigod\'s. He throws his enemies into the same blindness he experiences, then uses wisps of light to trick them into falling to their deaths. Towers of Light are less effective against Oculus because his Prism ability redistributes some of the damage he takes to anyone near him. Oculus\' initially weak sight range and low health make him an excellent target for ranged assassins.\n\nThe blind king, Oculus is an ancient and wise Demigod. His blindness is deceptive, as Oculus manipulates the sight of all those around him. He sits on a floating throne, leading his army into victory.',
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Now, we can assume that of the 3 unknown units, Oculus would probably be the "King" one (Throne). Due to some old concept art, I believe the other one has to be bird. Here is the (likely) concept art for Oculus and Bird Demigod:

