Complex sentence: Adopting the name "The Mistress", and using "Missy" as an alias, she uses a Matrix Data Slice as a "Nethersphere" to trap the conscious minds of the recently deceased as part of an alliance with the Cybermen.
Split into simpler sentences: Adopting the name "The Mistress", she uses "Missy" as an alias. She uses a Matrix Data Slice to host a virtual afterlife, trapping the conscious minds of the recently deceased as part of an alliance with the Cybermen.