"Elite" is a term for monsters and NPCs that require a group of players of about the mob's level to take down. Don't bother trying to solo one unless you are several levels above it. And if you are lower level than it, you are just dead. Accept it.
"Aggro range" is the distance from a mob within which the mob will notice you and attack. This is based on your level and the monster's level. If you are a higher level than the monster, the aggro range is smaller (if you are sufficiently higher level than a mob it won't even notice you if you stand right in it's space). If you are lower level than the monster the aggro range is bigger. Perhaps a LOT bigger. (That dragonkin in the east of Ashenvale came clear across nearly a whole, fully-pulled-out radar radius to come after me. Even with that head start I still couldn't run fast enough to get away. Sheesh)
Now, the problem here is, the Sepulchur area is a lowbie undead area (levels 11-15 or so) BUT Blizzard has this wandering Elite mob of level 24 (24!) in a area where lowbies are expected to hunt for several quests. He's so much higher level you are dead almost before you know what's attacking you. And he WANDERS, which means there's nowhere safe. And his aggro range for a level 14 is LARGE. You may have been safely hunting in one spot for 10-20 minutes when suddenly "RAAARRR" you are being attacked by this wolfman.
I have only leveled up a Tauren in addition to this Undead, and the Tauren lowbie area has no such hazard. I just don't know what Blizzard is/was thinking, putting that mob in there.