The good news for your project is that your reader's point of view is not under the influence of your emerging "bringer-of-insanity" creature. In fact, this is an opportunity for your narrator to become more sane... more clinically detached and detailed in their description of what is going on.
One of the most beautiful aspects of going insane is how beautiful insanity can be. It can engage all of the senses. The warping of character synapses into odd shapes can lead to very lyrical and tactile prose. It may be absolute chaotic misery for your characters but you as the writer can stand aloof and report the events in a calm and organized way.
As the beast emerged from shadow, all that was previously governed by light succumbed to its dark authority. New, more sinister aspects arose from familiar things. The pistol in Jane's left hand which had always felt good to her, heavy and powerful, now grew cold and hateful. Her arm ached with its hunger for killing. Longing for discharge, it sang seductively to her; begging for release.
William was no less affected. As the supportive presence of his teammates poured out of him like fresh shed blood, he found a burning cauldron of rage simmering at his heart. The smell of its sulfur assailed him. Jane's gun might be singing to her of its need, but his was screaming in fury. It wanted what hers wanted, but it demanded to go first. Obediently, William raised the savage steel to shoulder height with sights on his former friends.
This is also a great opportunity for grandiose statements about the nature of reality...
“The most merciful thing in the world, I think, is the inability of
the human mind to correlate all its contents. We live on a placid
island of ignorance in the midst of black seas of the infinity, and it
was not meant that we should voyage far.” ― H.P. Lovecraft
Have fun with your character's fragile souls. Those which survive will forgive you eventually.