How the hell do you even get into the Terathan Keep? There's an impenetrable wall of enemies there, and they spawn faster than can be killed. Even if you make headway, they spawn hyper-fast behind you as you move inward, and then you end up surrounded, and then you end up dead, and then you end up having to Help -> Stuck out.
I did it in Fel for that precise reason: you can go through Fire Dungeon => Despise => Tera Keep, and leave through Star Room.
Start the Mephitis champ spawn and use a high area damage char to quickly raise to level 2 (level 1 is only Spiders and Scorpions. I used a lvl 6 spellweaver with Essence of Wind and cleaned all the level 1 spawn in minutes), then bring the char you want to raise Loyalty with and do the level 2 (all non-caster Terathans). You can go in the corridors on the side, nobody will go there to bug you, but if you stay on the central area chances some PKs going from Despise to Star Room will notice you. Level 2 should take you about 20 mins solo and raise your Ophidian Loyalty by about 3k points, you will likely get some SoTs in the process as a bonus. If you have some time to loose, let the candle reset before they reach level 3, or when Level 3 spawns get out until next day, as too many Matriarchs and Dread Spiders packed in a small area will likely get you stuck and kill you. If you want some fun you can come back with a stealther and watch those PKs beeing slaughtered as soon as they drop from the Despise teleporter on their way to StarRoom hehe.
Overall I would say the entire process will take you 3 x 1 hour to reach to 10k Ophidian Loyalty. Don't bring anything you are afraid to loose in case you happen to cross a PK on the way, or in case you don't manage to get out when level 3 spawns.