Blood Castle

The Archangel had been quietly guarding the Continent of MU against Kundun's monsters, until Kundun's army overran his castle and wounded him. Without the Divine Weapon of Archangel he cannot recover his strength, so he calls the warriors of MU to sneak into the occupied castle and fetch it. You fight your way over the bridge, break into the castle, and the first player to carry the weapon back to him completes the event.

You need an Invisibility Cloak to enter. Since the Season 20-2 update there is one cloak for every level instead of separate +1 to +7 cloaks, and old numbered cloaks were converted automatically. It is made at the Chaos Machine:

Since the event-map renewal you register at the Event Map Square, with the NPC Archangel's Spirit. Open the warp window (M) and pick Event Map Arena, or click the Blood Castle icon that appears at the top left when the event opens; either way costs 20,000 Zen.

Your character level decides which Blood Castle you enter. The Season 21 update set these entry brackets, shared with Chaos Castle, Devil Square and Doppelganger, and rebalanced the monsters to match:

• Level 50 – 399
• Level 400 – 799
• Level 800 – 999
• Level 1000 – 1199
• Level 1200 – 1399
• Level 1400 – 1599
• Level 1600 and above

• Each Blood Castle level holds at most 10 players.
• Dying inside costs no experience, no Zen and no items.
• The warp command works inside Blood Castle.
• In the castle's safe zone you can still use support skills such as attack and defense buffs, heal, Soul Barrier and Swell Life.

• Every player still alive when the event ends receives one Jewel of Chaos.
• Large experience bonuses go to whoever breaks things and completes objectives, such as destroying the statue or returning the weapon.
• Characters at master level or higher can also earn a Mastery Box, delivered to the Gremory Case when the map closes. Its grade depends on contribution: monster kills count for the party, gate and statue destruction count for the individual.