Arnold had arrived on the planet with 16 of his summons and Nilerea who was his apostle. That was his entire army but within a couple of hours, his army had expanded to over 100,000 with the addition of the elves Tiamut had killed.
Their corpses were collected and dumped in front of Yan to do his lich magic and turn them into his loyal undead soldiers. Just like Arnold had thought, there were elves who had managed to save themselves using top tier elven magic. However, they had used all of their mana to protect themselves and now they were no different than a bunch of low levelled goblins.