An unexpected alliance.

As an otherworder, Armace is not from the world in which Slayers takes place. He comes from one of three other worlds that co-exist with the Slayers one, the world where Dark Star Dugradidgu and Night Dragon Vorpheed warred with each other for centuries. Armace explains to us that Dark Star was the victor in this battle but when it absorbed Vorpheed's essence, it went out of control and began to destroy literally everything in its path.

As a subordinate of Vorpheed, Armace is a shinzoku and along with his companions, he devises a plan to destroy Dark Star once and for all. This plan involves finding the five weapons of light that Dark Star itself created, two of which were hidden away in the Slayers world. Armace's plan is simple: open the gateway to his world to summon Dark Star, close the gateway so Dark Star cannot escape and then destroy him. To accomplish this, however, Armace needs to ally himself with someone familiar with the world in which the last two weapons remain hidden.

This is where Valgarv comes in. We know very little about why Armace approached Valgarv rather than anyone else. Is it because Armace knew that the ancient ryuzoku hid away the most powerful of the weapons? As the only remaining ancient ryuzoku left, Valgarv would certainly have some idea as to where the weapon may be hidden. This is never explained in the series as far as I can tell, but it's certainly plausible.

Armace offers Valgarv aid in getting his revenge against Lina Inverse in exchange for help in finding the remaining light weapons. Valgarv already knows that Gorun Nova, the sword of light, is being held by Lina's companion, Gourry. He sends his minions out to search for the other while he attacks Lina. What Valgarv wasn't expecting was that Lina would be allied with Xelloss, a powerful mazoku. Valgarv asks Armace to use the Ragudo Mezegis, the lance of light, to give him the advantage. Armace agrees but warns him not to cross blades with the Gorun Nova, as Valgarv has no idea of the true horrors that those weapons are capable of.

Valgarv doesn't heed Armace's advice at all and ends up crossing blades with Gourry, which creates a massive feedback of energy. Armace himself intervenes to prevent Dark Star from being summoned and Valgarv is furious that his battle is forced to end. Valgarv confronts Armace and demands to know why he must wait to kill Lina; Armace says he has nothing against helping Valgarv obtain his revenge but Valgarv swore to find the remaining weapons first. Furious, Valgarv can't deny that he swore an oath and reluctantly swallows his pride.

Of course Valgarv is impatient so he can't wait long before he takes Ragudo Mezegis for himself and goes to hunt down Lina. He successfully takes Gorun Nova for himself and prepares to summon Dark Star ahead of time, before Armace has gathered all five weapons. Valgarv sees Armace fraternizing with Lina and assumes that Armace has betrayed him, his anger fuelling his reckless behaviour. Of course, he doesn't realise that he's actually betraying Armace instead, as he's going against the shinzoku's plans.

Valgarv is successful in summoning Dark Star, with Armace helpless to do anything about it. Valgarv is engulfed in Dark Star's black energies shortly before Armace's companions arrive to close the gate. The relationship between the two was always one of convenience but I believe Armace became cross with Valgarv's early summons because he didn't want the people of the Slayers world to suffer Dark Star's wrath. I don't think Armace is happy to see Valgarv perish but I can imagine him being relieved to not have to deal with his reckless behaviour any longer.