[ Basically, he's trained at dealing with ghosts/demons/spirits, etc., I am also unfamiliar with Sebastian's canon beyond what little I've picked up in fandom.
the immortality bit is only a headcanon thing to give me an excuse for playing modern stuff, not an actual thing in canon (at least not a known thing)! I'm happy to use it, though! Lan Wangji plays a song called 'inquiry' every day for like 13 years, trying to reach someone, so perhaps Sebastian comes in during that time to serve him and tries to shift his attention to something else/away from the immortality thing? ]
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the immortality bit is only a headcanon thing to give me an excuse for playing modern stuff, not an actual thing in canon (at least not a known thing)! I'm happy to use it, though! Lan Wangji plays a song called 'inquiry' every day for like 13 years, trying to reach someone, so perhaps Sebastian comes in during that time to serve him and tries to shift his attention to something else/away from the immortality thing? ]