Cloak Games book 12. And The End. Serious spoilers ahead.
Lord Morvilind lands in Nadia's life again -- with a bus-crash -- shortly after Riordan had asked her to marry him. With him is an elf, called Tyth for short, whom Nadia had never seen before. She was Lord Morvilind's elf shadow agent, as Nadia had been his human one.
He has a plan. He realizes it needs Nadia's help -- and the scale of it is such that Nadia realizes she really needs to do it -- and Riordan, of course, will not let her go alone.
It involves attacks by Archons from gates, slave camps, discussions of smoothies and hats, Mars, quantum entanglement, an amulet Nadia stole much earlier in the series, and a lot more.