History: 

BTS-Long ago, Metarchus was one of the Blood, who fought against the evil of Zarathos. She may even date back as far as 20, 000 BC.
At some point, Metarchus turned from the Blood, and became one of the Fallen, who instead worshiped and served Zarathos.
The Fallen were banished to an unnamed dimension in the distant past, and Metarchus presumably was among them.

Centuries, or perhaps millenia ago, the Fallen assaulted a South American tribe (Incan? Aztec? Mayan?).

BTS - The Fallen regained access to Earth via the machinations of their own sorcerous ally, Salome'.

Several of the Fallen, including Metarchus, appeared in Cypress Hill Cemetary, rescuing Zarathos from defeat at the hands of the Midnight Sons.

Zarathos and the Fallen attacked the Midnight Sons briefly, then abducted Caretaker and invaded and took over Caretaker's former base below the cemetary. The Nightstalkers ( Blade, Frank Drake, and Hannibal King) followed them, but Drake was trapped and the others for forced to temporarily flee.

Patriarch joined the Fallen.  Metarchus and the Fallen observed as Patriarch and Zarathos plotted against the Blood and continued to torture Caretaker. 
The darkholder redeemers  sought out the Blood member Truthsayer, in the jungles of Central America. Metarchus followed and attacked them, seeking to either convert or slay Truthsayer. After some battle, Modred blasted and destroyed Truthsayer, apparently to prevent her from revealing the true nature of Victoria Montesi. He did so such that Metarchus, who had taken Modred's form at the time, was blamed for the murder. Metarchus escaped back to Zarathos' side.

Metarchus posed as Caretaker and drew the Midnight Sons in to "rescue" him, then attacked them, as well as Embyrre and Raydar, who had joined them. Metarchus was eventually overpowered, but managed to escape through a dimensional portal.

Metarchus listened as Patriarch explained their connection to Salome'.

Metarchus observed Zarathos' apparent killing of Ghost Rider/ Ketch.

Metarchus, alongside Zarathos and the remaining members of the Fallen, engaged in final battle with the Midnight Sons. They fled back to a ruined Tibetan monastery to regroup, and then met up with the Midnight Sons at the South American jungle temple base of Foundry. Ultimately, Metarchus fell before Morbius and Drake after being badly injured by Blade wielding Foundry's magical sword, Justicar. Blade then skewered Metarchus, apparently killing her. 

Skills: 

Metarchus is a metamorph, able to alter her shape into virtually any form imaginable. To some degree, she takes on the physical abilities inherent to a form she adapts. She claims to have the attributes of every monster in the history of the Earth. She is presumably immune to the effects of aging and conventional disease.
She wears a collar from Zarathos that forces her to obey his commands.