Other possible translation: snake witch.
First appearance with the third wave from Lich Shire Undead army in the plain of despair (v7c2).
The Serpa Witch is a high tier dark magic user and probably a female undead witch. Level over 300.
In Bar Khan previous war, "the Serpa witches were the ones in charge of the Undead legion’s vanguard". (v7c2). They are part of the Elite.
Leadership over high level Undead, like bone knight.
Necromancer magic (tier 1 and 2). Being able to raise more undead makes them even more dangerous on the front lines. v7c2
Dark magic & curses. They are amart and know how to protect and heal their mount. v7c2
Weakeness to silver, holy water and holy magic. Also, they have an "unusually low vitality " and in larger warfare, the undead giant rhino are vital to their own survival. v7c2
During the battle in the plain of despair (v7c2), 3000 serpa witches appeared. Boss-Class Witches may have been there but no special indication.
They were officers, "truly elite, (of) the Army of the Lich Shire" v7c2.
They have a crazy laugh on the battle field.
"More than 100 orcs warrior were necessary to kill one mounted witch".
The snake blood may imply some nasty unholy magical transformation.