From 1d4chan
What Alluminas might look like based on the descriptions.

If circumstances were right, I could probably be best friends with the other golden boy by now.

-Alluminas, contemplating his potential relationship with the Emprah

Alluminas is a Lawful Chaos God of the most absolute form of light, whose gaze renders other beings into unchanging, unmoving light. By nature, he is possibly Nurgle's opposite on one axis, and the arch-nemesis of Tzeentch on the other.

He is one of the least worshiped gods in the Warhammer mythos, since his physical appearance is an incomprehensible jumble of light and beams, while his creed and worship are equally beyond mortal understanding. So no statues, no texts, and no rapevikings of the sun. Even Tzeentch would be like, "Whoa there buddy! Even I won't go that far!" which puts things into perspective. He also has a brother, Obscuras, who bad control over shadow, and who was jealous of Alluminas and his light, so he joined to Chaos as Chaos God of Shadows or something edgy like that, real jealous little brother type shit.

Like most of the Gods of Law, very little is given about him since GeeDubs doesn't consider them hard and metal enough. In particular, Alluminas would never likely have gotten much elaboration as his entire schtick is "You cannot comprehend the form of Alluminas's Blessing".

Going by his description and appearance, he looks like a Naaru.

The Chaos Gods of Warhammer 40,000 and Warhammer Fantasy
Four Main Chaos Gods: Khorne - Nurgle - Slaanesh - Tzeentch
Other Gods of Chaos: Archaon - Hashut - Horned Rat - Nuffle
Malal - Morghur - Necoho - Zuvassin
Chaos Gods of Law: Alluminas - Arianka - Solkan the Avenger