How Do I Draw the Elves into the War?

Question

Background:

My evil Nature Mage (named Erilius) resides in a tropical forest where he has abundant access to local life (of which all the flora and fauna except people can be controlled by him) and where he has survived for 10,000 years through a combination of magic and his innate ability to control the minds of the sentient native cat people (to be known in the future as Felinus).

Note that Erilius is not human and is the last of a formerly benevolent humanoid race of sentient magic users who helped humanity by solving our lack of food problem. After humanity and the elves betrayed him and drove his race to extinction he fled across a great desert and built his tower in the Rainforest on the southern side of it. He vowed to one day get revenge and drive us all to extinction and has been keenly studying hundreds of thousands of scrolls and hoarding the knowledge of magic his people had achieved to keep it from humanity for the last 9,980 years of his 10,000 year life.

He has finally succeeded in uniting the too numerous to count competing tribes of the Felinus and has amassed an army of 3 million warriors (he united the entire race under his banner). He invaded the vast but disunified southern provinces of the human kingdom of Erial and has totally destroyed the Royal Mage Corps headquarters in the southern provinces.

The King (John the Wise) has amassed an army of 50,000 knights, 30,000 archers and 120,000 disorganized regular infantry armed with halberds, all types of sword, spears and pikes and 20 of the kingdoms most powerful mages (still weak compared to Erilius) and set out to meet him in battle on a great plain surrounded by hilly forest where the knights’ charge will be the most devastating and archers, infantry and siege equipment will protect the flanks. These soldiers are the combined allied forces of his kingdom and two other more northern human kingdoms. The elves, a magic-prone race are hesitant to get involved because they naively believe that Erilius is scared of them.

If the Elves (with an army of magic-wielding warriors combined forces with the human Mage Corps and the human armies they might be able to stand against this massive Felinus army and Erilius for long enough to evacuate their peoples to the mountains.

My Question:

What might be an effective way to reveal the light or the elves and convince them to join forces?

Linked (containing important details to my world that might or might not be useful for an answer)

• How to Escape a Horde of Worms Using Magic

• How to Defeat a Nature Mage

Answer 1

The easiest method I can think of:

Diplomacy.

Encourage the elves to exert themselves into posturing against the Mage. Have them send an otherwise vulnerable delegation of a couple 3rd level officials to speak with him. This sort of action is only taken by those who see themselves in a position of strength, whose very word operates as their shield.

There are two possible outcomes:

1. The Mage relents, sensibly noting that the combined forces might prove a valid threat to his conquest. He withdraws and the humans manage to escape. Humans win.
2. The Mage laughs and kills them outright. This reinforces the fact that he views them as weak and not a threat. The murder of the delegation is an open declaration of war, spurring the elves to ally with the humans. Humans win.

Good examples of this strategy (with the resulting outcomes) can be seen in lots of fiction (I'm using movies here simply for broad cultural awareness):

• Harry Potter and the Deathly Hallows; where Voldemort exposes himself after Harry's "death," unafraid due to his dominance.
• 300; THIS IS SPARTA!!!!! enough said.
• LotR: Return of the King; Sauron sends a single mouthpiece on horseback to taunt the hopelessly outnumbered attacking army. Aragorn cuts his head off in anger, "knowing" he's doomed and willing to die for his cause. If the ring hadn't been coincidentally been destroyed just then they would have all been slaughtered.

Answer 2

It could be that the elves would see the threat for themselves. You don't talk much about their psychology, but if they are the "close to nature" type, then a nature mage would be a natural enemy and a huge threat to their way of life.

Kind of a Ent vs Sauromon situation. The Elves are told what the mage is doing, see what the mage is doing for themselves, then the Elves decide that it's in their own best interest to help.

Even if they aren't the close to nature type, they could be convinced that after the humans are destroyed, they could be next in line.

OR, since they think that the mage is scared of them, then the humans could ask them to act as peacekeepers/negotiators with the mage. The elves agree because it shows them the humans look up to them and makes them look good, and then the mage shows he does not fear them at all.