Mortenax Blade is a 5-star Fire character on the Path of Nihility in Honkai: Star Rail. He scales off Max HP, spends his own HP to deal damage, and flips between a debuff-heavy support before his Ultimate and a full damage dealer once his Zone is up. Building him well means stacking HP and crit while keeping his Ultimate online so the Zone never drops.
How Mortenax Blade's kit shapes the build
Every damage number on Mortenax Blade is calculated from his Max HP, so HP is both his survival stat and his offensive stat. His Skill and Ultimate cost HP to use, and if his current HP is too low, the cost simply drops him to 1 HP instead of killing him.
His Ultimate, Fornax Ex Corpore, applies Balefire Bind to all enemies. That debuff cuts enemy DEF by 30% and raises the damage they take by 50% for 2 turns, then deploys a Zone and pushes him into the Infinite Fury state. While Infinite Fury is active he gains 20% Crit Rate and 60% Crit DMG, unlocks his Skill, and gets a new Ultimate, Tenax Per Ignem, that hits all enemies for a large multiplier of his Max HP.
The Zone is the heart of his loop. While it is up, ally attacks reapply Balefire Bind and feed him Charge. At 9 Charge he regenerates 25 Energy and fires a free extra Skill that counts as a Follow-up attack and does not cost HP. Keeping the Zone alive and his Ultimate available is the single most important thing your build needs to support.
Best light cones for Mortenax Blade
His signature, Reforged in Hellfire, is the clear top pick because it loads him with Max HP and hands him fixed Energy at the start of his turn once per wave. That Energy is what lets him cast his Ultimate on turn one and keep good Zone uptime. It also debuffs the target so he and his allies deal extra Crit DMG against it.
| Light cone | Why it works |
|---|---|
| Reforged in Hellfire (5★) | Best in slot. Big Max HP, once-per-wave Energy for early Ultimate, and a Purgatory debuff that raises Crit DMG taken by the target. |
| Lies Dance on the Breeze (5★) | Strong support option that adds SPD and cuts enemy DEF. Aim for 170 SPD through its passive, substats, and combat buffs rather than sacrificing other stats. |
| Holiday Thermae Escapade (5★) | Raises the damage of all his attacks and increases damage enemies take without hurting his support role. |
| Resolution Shines As Pearls of Sweat (4★) | Budget pick that applies a DEF reduction debuff. A toned-down stand-in for the SPD light cone. |
| Boundless Choreo (4★) | Good personal DPS choice thanks to Crit Rate and bonus Crit DMG against DEF-reduced enemies, but it gives the team nothing. |
Best relics and planar ornaments
The 4-piece Divine-Querying Master Smith set doubles down on everything his kit already does. It adds HP%, gives Crit DMG against DEF-reduced enemies, and grants the team a Comburent buff for extra damage once he applies the DEF reduction. Since he is the one reducing enemy DEF through his Ultimate, keeping the Zone active is what keeps the team buff flowing.
Pioneer Diver of Dead Waters is a reliable alternative. His kit applies debuffs constantly, so it is easy to hold the Crit Rate and Crit DMG bonuses, though the team loses the shared damage buff from the Smith set.
| Relic set | Effect summary |
|---|---|
| Divine-Querying Master Smith (4-Pc) | +12% Max HP; +28% Crit DMG vs DEF-reduced enemies; allies gain Comburent for +15% damage after he applies DEF reduction. |
| Pioneer Diver of Dead Waters (4-Pc) | +12% damage to debuffed enemies; +4% Crit Rate; scaling Crit DMG vs enemies with 2 or 3 debuffs, doubled briefly after he applies a debuff. |
| Longevous Disciple (4-Pc) | +12% Max HP; +8% Crit Rate per stack (up to 2) when hit or when HP is consumed, which he triggers often. |
For planar ornaments, pick based on the team. Bone Collection's Serene Demesne is the strongest damage option, while energy-focused sets help him fire his Ultimate sooner.
| Planar ornament | When to use it |
|---|---|
| Bone Collection's Serene Demesne | Best for damage. Adds HP% and, at 5,000+ Max HP, +28% Crit DMG. Ideal for a hypercarry build. |
| Lushaka, the Sunken Seas | Best when paired with an ATK-scaling DPS, adding +12% ATK to that ally plus +5% Energy Regeneration. |
| Sprightly Vonwacq | Advances his action so he can open with his Ultimate, plus +5% Energy Regeneration Rate. |
| Duran, Dynasty of Running Wolves | Good in Follow-up heavy teams; stacks Follow-up damage and grants +25% Crit DMG at 5 stacks. |
| Inert Salsotto | +8% Crit Rate and bonus Ultimate and Follow-up damage once Crit Rate hits 50%. |
Main stats and substats
Because his damage scales with Max HP, you stack HP wherever a crit slot is not available. The Body piece is where you choose between Crit Rate and Crit DMG depending on your light cone and current crit ratio.
| Slot | Main stat |
|---|---|
| Body | Crit Rate or Crit DMG |
| Feet | SPD (HP% if SPD targets are already met) |
| Planar Sphere | HP% |
| Link Rope | HP% or Energy Regeneration Rate |
| Substat priority | Crit Rate = Crit DMG > HP% > SPD |
Trace upgrade priority
Upgrade his Ultimate first since it powers the entire kit by deploying the Zone, applying Balefire Bind, and turning on Infinite Fury. His Skill is next as his main damage source, followed by the Talent that drives the Charge and Follow-up loop. The Basic ATK is last.
| Trace | Priority |
|---|---|
| Ultimate | Highest |
| Skill | High |
| Talent | High |
| Basic ATK | Low |
Eidolon value
His Eidolons reward investment without being mandatory. E2 stands out as the priority pull for teams built around Ultimate and Follow-up damage.
| Eidolon | Effect | Value |
|---|---|---|
| E1 | While the Zone is active, all enemies lose 20% All-Type RES; the Infinite Fury countdown is delayed 15% after the extra Skill. | High |
| E2 | Ally Ultimates count as Follow-up attacks and ally Follow-up DMG rises by 75%; his Charge cap drops to 7. | Priority |
| E4 | Heart, Refined ad Infinitum adds another +50% ally damage. | High |
| E6 | Extra Charge generation when he takes damage or spends HP, and Tenax Per Ignem's multiplier rises to 150% of the original. | High |
Best teams for Mortenax Blade
He fits any roster that benefits from constant debuffs, frequent attacks, or Follow-up damage. His steady Balefire Bind application and Nihility tag make him one of Acheron's strongest partners, speeding up her stack generation while satisfying her need for a Nihility teammate.
He also slots into Follow-up focused teams. Fast attackers such as Ashveil help trigger his Charge system more often, and Feixiao gains from both his debuffs and the team's high action rate. Any DPS that attacks frequently keeps his extra Skill activations flowing, which raises both his own AoE output and the whole team's damage.
His Technique attacks all enemies in range, Taunts them, and cuts his own damage taken by 90% for two turns, giving him a safe opening even before the Zone is set up. Combined with his innate Taunt and damage reduction inside the Zone, he can pull aggro for the team while still building toward his next Follow-up attack.
The throughline for every version of this build is simple. Stack HP and crit, keep the signature light cone if you can pull it, and never let the Zone fall off. Do that and Mortenax Blade covers both the support and damage roles at once, which is exactly what makes the Fire blacksmith so flexible across the current roster.