You can get to the [Eternal Throne] by returning to the starting point in the Gilded Arena, but it's much easier to select it from the world map (the above screen), even more so since there are no other places in the Gilded Arena worth checking out.
At the journey's end, go up the stairs and talk to the Chancellor. It will trigger a longer cutscene with the Lord of Bones. The game will automatically unlock a new main quest called [Phariseer]. As for the rewards for completing the current one, these are: 1000 XP, 5000 gold and the Interdiction ability (summoning Dead Lords).
[TIP: Acquiring Interdiction means you can now go to new locations. It's doesn't make any difference if you've only been completing main quests so far, but it's useful to know if you've tried exploring optional dungeons. It's also worth knowing that this ability isn't available at once. It will change only after defeating the first Dead Lord.]
[TIP: Before leaving this place, it's worth talking to the Lord of Bones again to explore all available topics. What's more, behind the Lord of Bones' throne there is a hidden Relic of Etu-Goth (the [Lost Relics] side quest)].