Beastmen. . . . Humans. . . Minotaurs. . . . Trolls, armoured in black full plate. . . . 25,000 fortified skeleton warriors armed in black and jade plate armour, each wearing an orb of ruby fire that glows and shimmers. . . . 10,000 vampires that ride swift demonic horses, and carry swords of hellfire, with a screaming wind of madness going before them!
It sounds like it's time for the Shadow Ravens to go to work sowing dissention in the enemy camp. Fortunately, there are several ways to go about this.
First, send all of the vampires a note saying that they're not welcome in Vallenorian land and, being uninvited, they'll have to go away
OK, since that won't work (although it should keep them out of your cities), use their presence to destroy the morale of the living units. Have Shadowraven infiltrators kill a few men, minotaurs, trolls, etc. through some method that drains their blood. Then leave them to be found by their companions. If you can, use magic so that they will register as having been killed by undead. (Maybe some kind of Undead Summoning spell would work for this). That way, the living units which can't be too comfortable with the vampire units to begin with will fear that they're being used as fodder for the vampires. Enough of this could start mutinies and infighting.
Furthermore, there are considerable numbers of different races in Gallanar's army. That should offer more opportunities for the Shadow Ravens to sow dissent. It will probably work even better because, being evil, they probably
are actually plotting against each other.
That's all delaying tactics though. In the end you'll still most likely be facing very powerful foes who outnumber you 5 to 1. If the Southern army is added to that, they'll outnumber you 8 to 1.
On the other hand, the Southern army only outnumbers the available central Vallenorian army 3 to 2. If you add in the Southern army, the numbers are about equal. Gethanen's army also has several notable weaknesses. Their only flying forces are 24 necromancers on demonic wyverns. And their only cavalry is mounted on triceratops. While Triceratops cavalry is very powerful, it is also quite slow--in fact the plate armored triceratops running flat-out move at a hustle for an unarmored man. (Their move is 30; adjusting for plate, 20--not fast, especially for cavalry). Their only fast troops are the tyrannosaurs which probably aren't too smart and the wyvern-mounted necromancers who are exceedingly few in number.
Gethanen's army has also been hiding underground for a long time and may be unfamiliar with terrain types and the warfare strategies of the much more open surface world.
This means that the Vallenorian army could use its griffons and other arial units to harass Gethanen's army at relatively low risk.
Gethanen's army is also open to hit and run manuevers. If the tyrannosaurs could be lured after a raiding party, they would quickly find themselves unsupported by their allies (all of whom they would quickly outdistance). Being both stupid and unfamiliar with the terrain of the surface world, it might well be possible to lure them into a narrow defile or some other area in which it would be possible to trap and slaughter them.
Gethanan's apparent lack of speed could also be exploited by the use of slow damage area effect spells. An acid fog or cloudkill would wreak havoc on the low and lumbering triceratops cavalry.
After hit and run harrassment and ambushes which kill the tyrannosaurs, it might be possible for the two Vallenorian armies to catch Gethanen's main army between the two of them and annihilate it. That would leave them only having to deal with Mallenar--a daunting task but much more doable with Gethanen out of the way and reinforcements arriving from the empire.