what happened to arhtas: after defeating illidan he walked up the frozen thrones stairs here he heard the voices of people like uther and murradin wich was his final test, then he struck frostmourne into the place wich it came from, the frozen throne wich completed "the sircle" then arthas picked up ner'zuls helmet and the souless death knight gained the soul of an orc warloc/shaman then they vere one. and the lich king (ner'thas ) could actualy when at full power crush atleast one of the aspects
Arthas did not kill Illidan. Illidan was simply mortally wounded for good and had to retreat back to Outland. Of course, he still resides in Outland, though the new expansion pack is not confirmed to actually bring him back.
Anywho, the Lich King, seeing as Arthas doesn't exist any more, is unliving. He's not dead, he's not alive, he's not undead, he's somewhere in the middle. He still has his humanity and stuff, but is all pale. But I don't believe he ages any more, just to add to it
A Death Knight getting his power from an undead orcish shaman, wielding a vampiric runeblade, and so great a threat that the demons put guards like Tichonderius there to make sure his master couldn't free himself from his imprisonment?