In the mission “The Return to Northrend” you should be able to slay Sapphiron & revive him as undead, but he´s not respawning & the ice pillars which should disappear once you defeated Sapphiron remain as well, so i couldn´t loot the treasure.
Also, of course, the sidequest is unsolvable in that case.
I defeated Sapphiron with cryptfiends only, without Arthas or other heroes.