Capital city invasions
Aside from the necropolis events outdoors, Scourge mobs will occasionally appear in Stormwind and Undercity. They spawn seemingly at random, causing city NPCs to yell for help. The most common are Flameshockers, level 55 mobs (level 63 elite mobs during the second Invasion) which do not hit very hard, but have a very large amount of health points. The city NPCs are capable of killing the Flameshockers on their own, but if a player or group chooses to tag them, they will receive 5 Argent Dawn rep (at Honored). Flameshockers can drop Necrotic Runes.
There are also groups of tougher elite mobs (Pallid Horror) that periodically spawn in Stormwind and Undercity and attempt to get to the city leader, causing him/her to yell that he/she is under attack. (These do not pose an actual threat to racial leaders, they are easily able to defeat the creatures if they manage to reach them.) When all the mobs are defeated, the city leader will yell that they are safe, and a dead Necrotic Shard will spawn. The shard will give a quest to talk to a NPC at Light's Hope Chapel. Any player can receive this quest, even if they did not tag the mobs that spawned the shard.
All the mobs that spawn inside cities are yellow (passive) and do not attack players who haven't attacked them first. They also appear not to attack vital NPCs such as bankers and vendors. So, it would seem players who aren't interested in the city events are not threatened by them in any way, and can simply ignore them and go about their business.