How did Kirito's Illusion actually kill? In Episode 19 - The Lugru Corridor, Kirito and Leafa are blocked from entering Lugru by Salamanders and are forced to fight with them. In the middle of the fight, Yui tells Leafa to use all her magic to heal Kirito from the damage of the incoming barrage of Fire Spells, while Kirito is chanting an Ilusion Spell that Leafa taught him when they entered the corridor.

Kirito turns into The Gleam Eyes from Sword Art Online and begins to kill everyone (save one Salamander who Kirito then bribes with the loot he just got, while another fled by diving into the lake, only to die).

Leafa explained previously that Spriggan's spells are mostly useless in battle, and by definition Illusions shouldn't damage enemy players or even kill them (unless they are intimidated and back off falling into an abyss of a monster filled lake). If they could do damage, they'd technically be summoning/conjuring.

Kirito isn't a hacker who made Leafa, Yui and all the Slamanders think that a giant beast was throwing them about, when really he was just stabbing them. And in Sword Art Online, Kirito didn't have any sort of summoning abilities (at least in the anime). So how did Kirito's Illusion pick up, throw and kill the Salamanders?
Rather than an illusion magic, it seems what Kirito used is a transformation magic instead. The following is based from the 3rd novel.

&quot;Well, I somehow remember the battle earlier. I used the magic Yui suggested, and became very big. Since my sword disappeared, I had to use my hands...&quot;
&quot;Also with the teeth bite yo~&quot;
On Lyfa's shoulder, Yui added with pleasure.

Remember that Yui is an AI from Aincrad and that upon arriving at the Alfheim world, Kirito noticed that Alfheim is based on Aincrad, including the skill sets and player data.

In the skills window, I saw several skills side by side. These skills included: 1-H Swords, Unarmed Skills, and Parry, the Combat skills, as well as Fishing, a support skill, but the proficiency values were abnormal. More were just about 900, and some had even reached 1,000 and had a tag denoting MASTERY.

Later when asking Yui, Yui confirmed that.

&quot;It seems this world is based on a copy of the Sword Art Online server.&quot;

This explains how Kirito can turn into The Gleam Eyes. Since it is a copy of the SAO, some resource file might be still left intact. While it is not normally accessible by the players, with Yui giving assist, it is possible that she hacked the system a bit and granted access to Kirito to transform into The Gleam Eyes. This wasn't explicitly mentioned in the novel, however.