Isla’s role was to dispose of the Vampire traitors. The majority of the expedition army were unaware that there were Vampires in Talosheim, so it was likely that the Vampire traitors would be able to escape. That was why Isla, who was borrowing a Magic Item that indicated the location of a blood sample’s owner, would hide in confusion and dispose of them.
She was also there to assassinate the evil Vampire hunter during the expedition, and most importantly, she was a trump card to ensure that the Dhampir, whom the evil god had even sent a Divine Message to command the Vampires to slay him, would be disposed of.

“That is why Ternecia-sama has sent me here,” she said. “I am a Vampire Countess, one of Ternecia-sama’s five strongest subordinates.”

Vampire Countess. A Rank 10 Vampire named after the court rank of count.

A-class adventurers would be needed to defeat such a Vampire, and even that would only apply if the Vampire were to stand and fight head-on. There was no small number of such powerful Noble-born Vampires hiding in Kyojou* that were even more dangerous than lower-level Dungeons, with many Subordinate Vampires in their command.

TLN*: I’m not sure what this term is supposed to be. In normal Japanese usage, it means “castle of a daimyo/feudal lord”, but that translation doesn’t fit here. I’ll update this word if the author ever bothers explaining what it is.

As a result, these Vampires were beings that could even turn the tables on a party of A-class adventurers. Even Gordan had never defeated a Vampire Count.

And including the Vampires dispatched by Birkyne and Gubamon, there were thirty Vampires under Isla’s command. The weakest of them were Rank 7, and there were even two Rank 9 Vampire Viscounts, only a single Rank beneath her.

They formed a large fighting force that would be sufficient to annihilate the entire expedition army on their own.