Borderlands 4 Horrors of Kairos Event: Mechanics, Raids, Rewards & Tips

The first live seasonal event in Borderlands 4, designed to inject replayability and global world activity into the early endgame cycle. Read our BL4 Halloween event guide, we'll break down the mechanics, raids, rewards, and tips.

Borderlands 4 Horrors of Kairos Event

Horrors of Kairos is Botherlands 4's first limited time seasonal mini event will scare all players senseless from October 23 to Novemeber 6. During the event, world boss encounters change the weather to a terror-inducing blood rain, with new legendary loot.

How Does Borderlands 4 Halloween Event Work?

During the event period, random weather events can occur in open-world zones. When these trigger:

The sky darkens and a terror-inducing blood rain begins. World bosses transform or spawn under this weather effect. Defeating them during the event yields exclusive legendary drops, cosmetics, and season-limited Borderlands 4 items.

So the idea is: you’re roaming the open world → a world boss appears → the whole area changes to a horror aesthetic → you fight it for themed loot.

This is a big design shift — it makes open-world exploration more rewarding, rather than just running instanced boss farms or drill sites.

Borderlands 4 Halloween Boss

Bloom Reaper is the first-ever Raid Boss / Invincible Boss in Borderlands 4. Designed as a large-scale endgame challenge boss — akin to the Invincibles from previous Borderlands games like Terramorphous, Voracidous, or Haderax. Launches alongside Ultimate Vault Hunter Mode 6 (UVHM 6), which raises the difficulty cap and adds new loot scaling incentives.

How to Defeat Bloom Reaper?

Pre-fight Mobbing:

You’ll battle hordes of high-level enemies en route to Bloom Reaper’s domain. This prevents “arena cheesing” and adds tactical progression.

Boss Arena:

Once inside, Bloom Reaper engages with massive AoE attacks and phase mechanics. Likely features environmental hazards tied to “bloom” or “reaping” themes — maybe corrupted flora, spores, or blood tendrils.

Loot Phase:

Defeating the boss grants proof of victory rewards (legendary drops, cosmetic trophies, etc.).

Possibly includes a new Echo skin or Vault Hunter ornament.

Borderlands 4 Halloween Event Rewards

Two brand-new legendary items debut with Horrors of Chyros:

1. Murmur — Legendary T.E.R. Assault Rifle

Executes weakened enemies with guaranteed crits when they’re below 35 % HP. Essentially a finisher-style gun that rewards precision and timing. Great synergy for crit-based Vault Hunter builds like Amon the Forgeknight or Rafa the Exo-Soldier.

2. Skully — Legendary Order Grenade

“Death-ding” grenade that seeks out the nearest target, firing projectiles as it travels. Functions almost like a mini-homing drone; potentially scales with grenade-damage or elemental bonuses. Very thematic for a Halloween event — fits the “skull chasing you” vibe. Both items drop only during this event (unless later re-added via Vault Card rotations or future events).

Additional Rewards

• Weapon skins

• Head skins/cosmetics

• Possibly limited titles or Echo-4 drone skins (if tied to event challenges)

Nothing in the leak indicates a new map or questline — it’s an open-world modifier event, not a story DLC.