Honkai: Star Rail 4.3 Mortenax Blade Build Guide: Relics, Teams & Eidolons

With the official release of Honkai: Star Rail Version 4.3 "The Lethe Below the Living", the highly anticipated 5-Star limited character Mortenax Blade has officially joined the roster.

As a Fire-Type character following the Path of Nihility, Mortenax Blade acts as a premier hybrid powerhouse. By deploying his unique Bounded Field, he seamlessly blends team-wide buffs, massive DEF Reduction, and Vulnerability application with impressive personal damage. Whether you are building a meta Acheron setup, investing in the new Ashveil Follow-Up Attack (FUA) archetype, or searching for a future-proof support unit, Mortenax Blade is easily one of the most high-value pulls of Version 4.3.

In this ultimate guide, we break down his official kit, Eidolon priorities, optimal gear setups, and Tier 0 team synergies to help you spend your Stellar Jades wisely.

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1. Skill Mechanics & Kit Overview

Mortenax Blade stands out from traditional Nihility characters by utilizing an HP-scaling survival model alongside an incredible multi-multiplier team amplification system. His core utility heavily revolves around his Ultimate (Fornax Ex Corpore), which deploys a special Bounded Field zone that applies devastating debuffs and enhancements:

Trace / Debuff Type	Core Buff Value	Mechanics & Special Conditions
Enemy DEF Reduction	30%	Natively shreds enemy defenses, directly multiplying your team's raw damage output.
Vulnerability	50%	Multiplies the final damage enemies receive—a rare, independent multiplier zone.
Universal DMG Boost	50%	Provides a generic DMG amplification that works seamlessly with any Main DPS.
Additional DMG Modification	Up to 75%	【Core Choice】 If another Nihility character is present, this grants a massive 75% Ultimate DMG increase for allies. Otherwise, the bonus converts into Mortenax Blade's personal damage.

Core Strengths:

HP-Based Bruiser Utility: Unlike most Nihility units that scale off ATK, Mortenax Blade thrives completely on Max HP. His built-in Taunt mechanics and self-DMG mitigation grant him unparalleled survivability.
Dual Metas Catalyst: He single-handedly enables rapid stack generation for Acheron while acting as the premier dual-DPS partner for the Ashveil archetype.

2. Pulling Strategy: Is He Right For You?

Your pulling priority for Mortenax Blade should align with your current character roster:

1. Highly Recommended 🌟🌟🌟🌟🌟

Acheron / Ashveil Main Players: He consolidates multiple debuffs into a single slot. At E0, he completely unlocks their full damage ceilings while drastically lowering team-building friction.
Follow-Up Attack (FUA) Enthusiasts: Excellent in high-frequency FUA setups (such as Feixiao, Topaz, or Tribbie setups), where his passive hit-triggers maximize overall buff uptimes.
New Players Lacking 5-Star Supports: His universal DMG amplification and robust survivability make him an incredibly future-proof account investment.

2. When to Consider Skipping ⚠️

Strict Future Character Savers: If you do not own Acheron, Ashveil, or any dedicated Follow-Up ATK cores, you can safely skip him and save your Stellar Jades for future banners.

3. Eidolon Priority & Budget Roadmap

According to rigorous community testing, Mortenax Blade's Eidolons generally yield a significantly higher combat return than his signature Light Cone.

$$\text{Recommended Pulling Progression: } E0 \rightarrow E1 \rightarrow E2 \rightarrow E2S1 \rightarrow E6$$

1. Official Eidolon Value Breakdown

Eidolon Level	Official English Name	Impact & Breakthrough Analysis
E0	Base Character	Complete Kit. Grants the baseline DEF shred, Vulnerability, and universal buffs. Easily matches limited Harmony utility at base. F2P friendly.
E1	Ere My Death, I Stood Unmade	Core Upgrade. Decreases all enemies' All-Type RES by 20% while the Zone is active, introducing a rare RES PEN multiplier and optimizing Ultimate uptime.
E2	Ash Was My Heart, Yet the Flame Stayed	Game-Changing Quality of Life. Ally Ultimates are treated as Follow-Up Attacks. Massively boosts FUA damage while reshaping his energy mechanics into a self-sustaining core.
E4	Odium I've Smitten, Thence Came the Blade	Heart Refined ad Infinitum Amp. Adds an additional 50% team damage bonus while the Zone is active, significantly pushing your endgame damage ceiling.
E6	If Immortality Could Die, I Swore THEM Slain	The Ultimate Form. Automatically triggers Charge generation whenever he takes Damage or consumes HP. Accelerates Ultimate cycling while maximizing his personal scaling to a 150% independent multiplier.

2. Spending Breakdown

F2P Players: Stick to E0. Use accessible 4-star Nihility light cones for great cost-efficiency.
Dolphins / Monthly Pass Users: Target E1 to unlock All-Type RES PEN for your entire squad.
Meta Min-Maxers: Aim for E2 or E2S1. E2 redefines his archetype scaling, and S1 covers all remaining personal DPS requirements.
Whales: Head straight for E6 to experience zero-cooldown, infinite Ultimate rotations.

4. Best Relics & Planar Ornaments Setup

Remember: Mortenax Blade gains zero benefits from ATK stats. All personal damage and scaling mechanics thrive completely off his Max HP.

1. 4-Piece Relic Sets

Forge-Themed Version 4.3 New Set [Best in Slot]:
- Mechanics: Tailor-made for his HP-scaling and Crit scaling toolkit. Inflicting DEF reduction triggers additional team-wide damage modifiers, perfectly balancing his personal sub-DPS with group utility.
Eagle of Twilight Lineage (4-Piece): Tailored for advanced high-speed setups, utilizing the Action Forward mechanic to unleash his Ultimate before your Main DPS takes action.
Guard of Wuthering Snow (4-Piece): Enhances defensive metrics and self-sustain when running high-difficulty content alongside heavy shielders.
Pioneer Diver of Dead Waters (4-Piece): A strong alternative if you are shifting Mortenax Blade into a dedicated personal DPS role alongside multiple debuffers.

2. 2-Piece Planar Ornaments

Sprightly Vonwacq (★★★★★): Grants 5% Energy Regeneration Rate. Advances Action Forward by 40% at the start of battle if Speed $$\ge 12$$. Preferred choice for immediate turn-one Ultimate deployment.
Bone Collection Domain / Peaceful Bone Burial Ground (★★★★★): Max HP +12%. Drastically increases Crit DMG for himself and his summons when crossing the 5,000 Max HP threshold. The premier choice for a DPS-focused build.

5. Light Cone Hierarchy

1. Signature Light Cone: Reforged in Hellfire (S1)

Base Mechanics: Increases Max HP by 30% and regenerates 20 Energy at the start of the wearer's turn/wave. Basic Attacks apply specific debuffs, reducing target Crit DMG by 30% while amplifying Mortenax Blade’s own Crit DMG. Strong weapon, but E1 and E2 offer much higher vertical value for your account.

2. Viable Alternative Substitutes

Support-Oriented Build: Before the Tutorial Mission Starts (S5) ＞ Resolution Shines As Pearls of Sweat (S5) ＞ Incessant Rain / Those Many Springs
Damage-Oriented Build: Along the Passing Shore (Acheron Signature) ＞ Boundless Choreo ＞ Good Night and Sleep Well

6. Meta Team Compositions

Thanks to his massive multi-debuff uptimes and generic scaling multipliers, Mortenax Blade easily drives Version 4.3's strongest team variations:

1. Ashveil Dual-DPS Team (Tier 0 Meta)

Lineup: Mortenax Blade + Ashveil + Tribbie + Sustain
Analysis: Mortenax Blade and Ashveil establish a highly destructive dual-DPS Follow-Up core. High-frequency attacks comfortably sustain peak buff uptimes, marking the pinnacle of 4.3 FUA performance.

2. Acheron Hyper-Buff Comp (Ultimate Stack Catalyst)

Lineup: Mortenax Blade + Acheron + Sparkle + Lingsha
Analysis: Mortenax Blade rolls DEF shred, DMG amp, and Vulnerability into a single slot. He functionally optimizes Acheron's Ultimate stack requirements while leaving room for premium Harmony buffers.

3. Pure Follow-Up ATK Team

Lineup: Mortenax Blade + Feixiao + Tribbie + Sustain
Analysis: Capitalizes heavily on high-frequency team actions. Excellent synergy through multi-hit Follow-Up ATK mechanics and universal team buffs.

4. Mortenax Blade Hypercarry Setup

Lineup: Mortenax Blade + Sunday + Tribbie + Sustain
Analysis: Shifts Mortenax Blade into the primary damage slot. Sunday and Tribbie continuously funnel actions, buffs, and energy into him, allowing his HP-scaled multipliers to shred enemy waves safely.

How to Get Oneiric Shards Efficiently: A Smart Tip for Players

Because Mortenax Blade’s Eidolons are so fundamentally impactful, hitting his E1 or E2 breakpoints is a top priority for most players looking to sweep Version 4.3's endgame content. However, direct in-game purchases of Oneiric Shards can often be overly expensive due to platform surcharges and premium regional pricing.

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Don't let a sudden resource bottleneck hold your account back. Unlock the true potential of a fully-built Mortenax Blade today, and breeze through Version 4.3's toughest endgame challenges!