Dear Playgrounders,

My players have decided they would like to do a sort of one-shot PvP battle with future versions of their characters. They're currently lvl 5 (druid, paladin, monk & bard) and will be adding five levels onto their character, possibly multi-classing. They also get to chose a rare magic item, though requiring approval by me as the DM.
Having never designed a Pvp arena before, I could use some help. For the setting, I was thinking of using this to introduce a bit of background lore for the setting through the arena. I currently have my mind set on a tomb hidden underground a the base of the river Styx. I was thinking several stone platforms with water from the river Styx flowing between them, effectively creating very small islands. They will have been brought there by Charon on his ferry. In the tomb, an ancient necromancer has hidden his fabled bone crown, with which he had mastery over life & death. Having found the crown, an ominous voice announces that only one may possess it. I'm certain several players will desire it, thus resulting in OOC-desired PvP.
The main problem I'm having is making the arena interesting to fight in. Any ideas would be greatly appreciated. I've currently thought of the following:
- When in contact with the river, it will deal 1d8 necrotic and 1d8 cold damage
- Possibly a water weird attacking the players from the water in a randomly determined location
- Skeletons or specters crawling out of the river
- A random platform suddenly starts sinking
- Brasiers with black flames
- A central platform containing an altar with the bone crown

Please fire away with your suggestions. I could really use them!