Here is an overview of the downloadable content, via its Steam page:

About

“Legacy of the Moonspell” is the first downloadable content for Vampire Survivors, and introduces a brand new stage, new characters, new monsters, and new weapons to slaughter them with. There might even be some hidden mysteries for those of an inquiring mindset, but we can neither confirm nor deny the presence of vampires…

The Fall of Moonveil

In eastern lands, a clan has fallen. The Moonspell, once vigilant guardians of a sorcerous valley nestled in the mountains, have been overrun by hordes of yokai and oni. Though treacherous, this hive of spectral activity may provide some clue as to the location of a vampire. If not, at least it’ll be entertaining to defeat thousands of wayward spirits in the process.

New Content

• Eight New Characters
  • Including:
    • Miang Moonspell – The last disciple of the Moonspell Clan before its downfall, and possibly its finest. Self-consciously a shounen protagonist.
    • Menya Moonspell – One of the few surviving Moonspell elders, Menya’s mystical powers are near god-like.
    • Syuuto Moonspell – Banished practitioner of the new moon dark magic, Syuuto is nonetheless an enemy of evil. Poor personal hygiene may contribute to his continued exile.
    • Babi-Onna – Returned from the dead to seek vengeance on demons and mortals alike, she nonetheless retains her impeccable ability to charm, amuse, and dazzle.
• 13 New Weapons
  • Including:
    • Silver Wind – An ancestral force unleashed by the staff of the Moonspell Clan, only those born under the moon can fully wield its power (or whoever finds it in a chest, we’re not sure).
    • Four Seasons – A set of orbs which unleash the power of the changing seasons, of death and rebirth.
    • Summon Night – A weapon which drips with the darkness of the new moon. Despite the clan’s superstitions, even the shadows can oppose evil.
    • Mirage Robe – An enchanted kimono, weaved on a loom from the silk of the earth spider, and imbued with a fragment of Babi-Onna’s vengeful spirit. Besides, it looks fabulous darling.
• A Brand New Stage
  • Mt.Moonspell – Our biggest stage yet, a sprawling map featuring several different environments, all with their own challenges and resident monsters! This one stage includes an abandoned castle, snow covered mountain, and a yokai infested village, and more besides.
• Six New Music Tracks
  • “Legacy of the Moonspell”
  • “Festive Breeze”
  • “Rumble Kachara”
  • “Haze of Night”
  • “Hell Night”
  • “Crystal Allure”

Vampire Survivors is available now for Xbox Series, Xbox One, and PC via Steam and Microsoft Store. It is also available via Xbox Game Pass.