Wasn't his attempt to garner the support of Mehrunes Dagon and invade the Battlespire a previous attempt to take over Tamriel? I think it took place before Arena, but it may have been simultaneous, I don't know.
Either before or during Arena, yes. To quote a conversation from Battlespire's Storyline:
Sirran Angada: Listen, knave, and cease your prattle. My Lord, Jagar Tharn, the mortal Emperor of Tamriel, and your Lord Dagon are ordering affairs high above your counsel. Tell someone in authority that I MUST speak with his Lordship.
Sinder Velvin: What? I'm not up on current affairs, but isn't the manling Emperor called Septic or Septem or September or something?
Sirran Angada: Gooood. Gooood. You CAN think, after a fashion. Yes, Tamriel BELIEVES that Uriel Septem still rules, but in fact, my master, Jagar Tharn, has taken the form of the Emperor and rules in his stead.
Sinder Velvin: That is VERY interesting. But what has that got to do with you and Lord Dagon?
Sinder Velvin: What? I'm not up on current affairs, but isn't the manling Emperor called Septic or Septem or September or something?
Sirran Angada: Gooood. Gooood. You CAN think, after a fashion. Yes, Tamriel BELIEVES that Uriel Septem still rules, but in fact, my master, Jagar Tharn, has taken the form of the Emperor and rules in his stead.
Sinder Velvin: That is VERY interesting. But what has that got to do with you and Lord Dagon?
The exact time of the Battlespire fiasco isn't given, but based on this quote it is clear that the events are happening during Jagar Tharn's rule of Tamriel.
EDIT: Damn, proweler beat me to it. I blame the crappy internet connection I currently have and the long amount of time it took me to find the quote.