WARNING: The following contains spoilers for X-Men #13 and A.X.E.: Judgment Day: Death to the Mutants #1, now on sale from Marvel Comics
Judgment Day has been a brutal event for the X-Men and the rest of the mutant nation Krakoa, with numerous casualties already coming at the expense of the Eternal assault. Luckily for them, a handful of Eternals has resisted that directive.
As revealed in X-Men #13 (by Gerry Duggan, C.F. Villa, Matt Milla, and VC's Clayton Cowles) and A.X.E.: Judgment Day: Death to the Mutants #1 (by Kieron Gillen, Guiu Vilanova, Dijjo Lima, and VC's Travis Lanham), Ikaris, Sprite, Kingo, and Gilgamesh are actively working with the mutants against the rest of the Eternals -- but that might change if the X-Men make a deadly mistake.
The X-Men have found themselves isolated from the rest of the Marvel Universe, even before the events of Judgment Day positioned the Eternals against them. Under the orders of Druig, the Eternals wage a massive assault against Krakoa and Arakko, resulting in untold death on the latter and chaos on the former. The Avengers quickly rise to the occasion, but the ecological disasters caused by the attacks divert their attention. But the efforts to stop Druig come down to an alliance between Ajak, Iron Man, (a kidnapped) Mister Sinister, and the other Eternals belonging to Ajak's group who are able to craft a new Celestial to cease the conflict.
Even before this, Ajak's allies establish themselves as opposed to Druig's plans and actively work alongside the mutants. As detailed in A.X.E.: Judgment Day: Death to the Mutants #1, the X-Men's efforts to repel the Eternals end up utilizing a joint offensive with the Eternals' attack against their own race. Ikaris, Sptire, Kingo, and Gilgamesh join in the battle, striking directly at Druig's plans for the mutants.
Working in psychic tandem with Jean Grey, Ikaris keeps her informed of their assault. While the rest of the team serves as a distraction, Sprite is able to override the defenses of the Eternals armories allowing Magik and the rest of the mutants to arrive on the scene. Ikaris then resists his typical noble impulses to strike hard at the powerful psychic Zuras, allowing Krakoa's telepaths to go unchallenged by his mental might. This gives the mutants a better chance to stall the terrifying Hexes long enough for the new Celestial to be born.
Even before they leave Druig's domain, they make sure to impart a message to the rest of their people claiming "Death to the Eternals" and staking their place in the conflict. This seems to truly cement the idea that the primary Eternals -- Ikaris, Sersi, Sprite, Phastos, Gilgamesh, Kingo, Ajak, and Makkari -- have wholly thrown their lots in with the X-Men. This coupled with the Avengers openly trying to assist the mutants might lend some hope to a possible alliance between Earth's heroes. If the entire hero community can stand united in the face of Judgment Day, they might sway the Celestial enough to ensure the world's survival.
This makes the Eternals' decision to fight alongside the X-Men an important one. Their knowledge and skills will be crucial in fighting back Druig's schemes, especially if he decides to double down on his assault against Krakoa. But things might not work out if the X-Men get any Eternals killed, something Ikaris notes in X-Men #13 will result in them breaking that alliance. Considering the warpath that Magneto is likely going to be on in response to Uranos' slaughter of Arakko, this vital bond might be broken before long, costing the mutants a major ally that's already helped keep them all alive.