The dialogs clearly tells that she is a DK. Just look at the spells that she is casting:
" In the Eastern Plaguelands version of Acherus, Lady Alistra summons random creatures from the shadow realm. While casting the summon, she will say one of the following lines:
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Lady Alistra says: Moving on…
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Lady Alistra says: Dearest shadow realm, grace us with the presense[sic] of one of your many denizens.
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Lady Alistra says: What beast lies in store for us…
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Lady Alistra says: Let us coldly greet another inhabitant of the shadow realm…
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Lady Alistra says: There are many lessons left to learn, let us move onto our next subject…
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Lady Alistra says: Let us see what atrocity we can pull from the shadow realm this time…
The interaction that follows will depend on what she summoned.
Shadow Constructs:
Lady Alistra says: Skeletal mages. A perfect opportunity to display the utility of some of our powers.
Lady Alistra says: Your spells will not be as potent as mine, but you will find anti-magic shells to be entirely invaluable against a large number of magical attacks.
Lady Alistra says: The power you absorb from spells with your shell can be used to counter with your other attacks.
Lady Alistra says: Mages fear our kind… with reason. Play upon this fear, disciples.
Forgotten Servants:
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Forgotten Servant says: Me… back… master…
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Forgotten Servant says: Me come back… master. Me… loyal servant.
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Forgotten Servant says: Sorry… me gone… so long… master.
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Forgotten Servant says: Finally… home…
Lady Alistra says: Ghouls? We have quite enough of those already.
Lady Alistra says: I often find the trouble with ghouls is that they outlive their usefulness too quickly. Let us let death and decay reclaim these.
Alistra casts Death and Decay on the three ghouls.
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Forgotten Servant says: Master?
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Forgotten Servant says: Me… bad?
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Forgotten Servant says: It hurts…
Lady Alistra says: At times, their corpses can prove more useful than the ghouls were themselves.
Alistra casts [Corpse Explosion], killing several Disciples of the Unholy.
Lady Alistra says: Hmm…
Lady Alistra says: Unfortunate.
A Cenarion Scout:
Cenarion Scout says: Wha… where am I?
Lady Alistra says: A druid! How curious… I wonder how it got in there.
The scout enters Tree Form as Alistra starts attacking it.
Lady Alistra says: Disciples… you will find with time that many healing spells fall victim to the degeneration of our strikes.
Alistra finishes off the druid.
Lady Alistra says: Some healers lack the knowledge to counter the diseases that we master.
Lady Alistra says: This lack of knowledge is just one more weakness among many. There is little hope against the power of death itself."
She raises ghouls, skeletons, casts Death and Decay and Corpse Explosion, this clearly gives you the hint that she is a hunter…