Can anyone solve this P99 forum puzzle?

[Swish]

Okay, time for another...

This place I guard as well as I can,
Helmet on head and sword in hand.
Outward I stare, unable to see
Invisible rogues or casters going past me.

Inside they quest for epics and treasure,
My brothers they slay for wealth you can measure.
Some might say I'm bad at my job,
Unable to deal with invisible mobs.

This city is under continuous invasion,
But I stay satisfied killing mobs of undead persuasion.
Leading the others is a difficult task,
Especially when you can't tell who's wearing a mask.

Who am I?

[Swish]

Posting again. Is everyone sure they've read it thoroughly?

Originally Posted by Swish [You must be logged in to view images. Log in or Register.]

Okay, time for another...

This place I guard as well as I can,
Helmet on head and sword in hand.
Outward I stare, unable to see
Invisible rogues or casters going past me.

Inside they quest for epics and treasure,
My brothers they slay for wealth you can measure.
Some might say I'm bad at my job,
Unable to deal with invisible mobs.

This city is under continuous invasion,
But I stay satisfied killing mobs of undead persuasion.
Leading the others is a difficult task,
Especially when you can't tell who's wearing a mask.

Who am I?

Perhaps worth saying this isn't the most popular city for guard slaying, but I do still see it go on.