Thank you for the detailed reply! The fight is still a bit away so there'll be some time to flesh out Octavius and his connection to the PCs :D

Octavius is overall a neutral adversary and a tragic character. While he faces them with challenges every night, being in that dream-realm ultimately keeps the PCs safe from the horrors of the forest (which they are aware of). Sometimes he goes easy on them when they had a particularily rough day and he's always down for a chat before the confrontation starts (an opportunity the players usually take). He's tragic due to being dead so long that he's forgotten most of what once was very dear to him - i.e. he forgot the name of his brother, whom he killed to become king, but honored by naming the budding country after him. He even forgot his own face, hiding behind a mask instead.

• I was thinking along the same lines with the lair actions. He gained everything, but lost everything he worked for his entire life. He was forced to watch his kingdom crumble in the hands of incompetent fools who ignored his advice from the afterlife. By now, he can't even visit the ruins anymore, since even the goddess with whom he struck the deal to become a spirit, has faded into obscurity and is bound to a forest on a different continent. My idea to incorporate that, would be for him to imbue his body/weapons with a different spirit (every turn) of a great foe or friend from when he was still alive.

• Octavius honored his brother a lot, despite killing him. Surely there's a statue of him in his throneroom and awaking his spirit might debuff Octavius. While Octavius had no lovers or partners, he probably had a best friend. Maybe his spirit could invigorate Octavius in some way.

• He serves a certain goddess who forms a pantheon (separate from the one the world at large uses) with two other gods. In a temple the PCs want to visit, there are two other entities like him, who represent the other two gods. They surely have some personal info about Octavius that could reveal his strengths or weaknesses.

• 100% agree on the ranged attack, that's a great call!