[Please note: I do not consider this part of Evaris' 'cannon' so much as a means to introduce him to The Inquisition for the purpose of RP.]
Simply put, The Breach drove Evaris completely mad. Having a demon in your head when there is a hole ripped through to The Fade will do that to an elf. He spent some time trying to find solitary places to hide, but it proved fruitless. With the war spreading like wildfire, everyone was scrambling to the secret corners of Thedas in hopes of keeping themselves an their loved ones safe.
Evaris did not wish to hurt any innocent people with his madness.
So he searched out The Inquisition at Haven. They were the only ones trying to deal with this Breach situation, so if anyone would find a way to help him, it would be them. If they judged he was too far gone and he needed to be put down, then perhaps it was true.
But Someone[*] decided he was more useful kept alive. This could have been because although Evaris is no mage, he is indeed possessed. A unique case, at the very least, and someone may have wanted to study how this came about.
So Evaris was locked in the dungeon beneath The Chantry at Haven. A few people may have even seen him there, raving mad and consumed in a thin layer of crackling green flame that bubbled and peeled his skin... His demon can see into the Greed of others, and sense what it is they feel they will never have enough of. He probably said some pretty horrible things to anyone foolish enough to pass by...
But everything changed when The Breach was closed. Evaris was more or less in his right mind, and Someone[*] let him out of his cell when Haven was being attacked. Faced with the sudden crisis, Evaris kind of accidentally fell into rescuing people from the dragon's attack, and helping them flee to the mountains. He had no where else to go, and no one out right told him to leave... So he just traveled with them.
All the way to Skyhold. And still, no had out right kicked him out. Staying one day after another just turned into Staying, somehow. He isn't even opposed to doing favors for The Inquisition, provided he can A) Stab some things or B) get paid.
He does enjoy slithering around and causing havoc though. Stealing things. Pranks. Drunken misconduct. He's likely also banged every servant boy/girl not too afraid of his fangs to give it a whirl.
Whee, for The Inquisition!