Hello!
You have opportunity to kill Horestis even if you missed to kill him once he appeared in his tomb. You can enter teleport to the Boss only when The Mummy's Curse World Change is active, so all jars needs to be broken again.
"Every time you break a jar you are granted one chance to face Horestis, and these chances can be taken as far in the future as you wish." - Source: https://tibia.fandom.com/wiki/The_Mummy%27s_Curse_World_Change
I have also 2 tips regarding killing Horestis.
1. If you manage to break more than 1 Ornate Canopic Jar, you can enter the teleport to boss more than once, as long he's in his Tomb. So if you break 2 jars - you can kill Horestis twice during the same World Change, or keep your "boss point" for next time.
2. You can wait for Double Loot Event with your boss point. "If you have managed to break a jar from the The Mummy's Curse World Change, then save the access point for these events. Horestis always drops a Scorpion Sceptre and will drop 2 of them during the event." - Source: https://tibia.fandom.com/wiki/Double_Loot_Events