1. The suffering of the people in the Empire of Blood has to come to an end. The current regime will be altered or replaced.
2. None of the numerous sadistic wishes regarding Tarquin's fate that were expressed in this and some other threads will be fulfilled.
Elan does have a plan and since he is thrilled and happy about it I don't think it involves Tarquin's death, or much violence. But I have no idea what could it be, or whether it will actually work the way Elan intends.
On that matter,
Do I understand correctly that you believe Tarquin's subplot needs to end before Xykon's main plot? Because I don't think so and I expect that Tarquin will be dealt after Xykon, when the Gate threat is over and the remaining protagonists return to the desert to deal with unfinished business.And when a fictional story introduces an 'evil', it needs to describe the defeat of that 'evil'. (See the quote in my signature for another way of putting it.) In the case of OOTS: no matter what the cost - even if the whole of the OOTS dies tomorrow, the story isn't over until Xykon is beaten, even if that takes another 30 years and 10 more adventuring parties. If Rich chooses to 'end' the story before then, we'll all assume he's just setting us up for a part 2.
And having introduced Tarquin, and gone out of his way to establish that he's 'evil', now Rich has set up the expectation that Tarquin has to be beaten as well. It's a subplot, and the main plot won't be fully resolved until the subplot is too. (There's also the 'redemption' option for Tarquin, but personally I'm writing that off as implausible.)
Consider The Lord of the Rings - the main quest was finished and the big bad was completely defeated midway through the book 6. Yet the story continued for several chapters.