Mortenax Blade is a 5-star Fire character on the Nihility path in Honkai: Star Rail, and he plays nothing like a standard debuffer. He spends his own HP to deal damage, drops a Zone that weakens enemies, and flips into an aggressive damage state once his Ultimate goes off. The catch is that all of his stats scale with Max HP, so the build looks very different from a typical attack-based DPS.
How Mortenax Blade works
Before his Ultimate, Mortenax Blade behaves like a support. He inflicts "Balefire Bind," which cuts enemy DEF by 30% and raises the damage they take by 50% for two turns. He then spends 30% of his Max HP to deploy a Zone and enters the "Infinite Fury" state.
Inside Infinite Fury, his CRIT Rate goes up by 20% and CRIT DMG by 60%, his Skill unlocks, and he gains the new Ultimate "Tenax Per Ignem," which deals Fire damage equal to 300% of his Max HP to all enemies. His Skill, "A Rain of Blades Seals Fate," costs 15% of Max HP and hits the field for AoE Fire damage that also scales off his Max HP.
The loop that ties it together is his Talent, "All Karma Comes Due." While the Zone is active, every ally attack applies Balefire Bind and grants him 1 Charge. At 9 Charge he regenerates 25 Energy and casts his Skill one extra time for free, counted as a Follow-Up attack. That is why fast-acting allies and Follow-Up attackers feed him so well.
One survivability note worth knowing. If a killing blow lands while he is in Infinite Fury, he is not knocked down. Instead, the Zone disperses, he leaves the state, and he heals 50% of his Max HP. He also can only use his Skill while inside Infinite Fury.
Best relics for Mortenax Blade
The best-in-slot cavern set is Smith's Fire Beast Mask. It gives him the two stats he wants most, HP% and CRIT DMG, and its 4-piece bonus pushes a 15% team-wide damage buff through the DEF reduction he constantly applies. That doubles as a personal CRIT DMG boost against DEF-reduced targets.
If you don't have it built, Pioneer Diver of Dead Waters is the next pick. His personal damage barely drops, but the team loses the supportive buff, which makes it weaker overall. Longevous Disciple is also a stable option since he constantly consumes his own HP, keeping the CRIT Rate stacks active without setup.
| Relic set | 2-piece | 4-piece |
|---|---|---|
| Smith's Fire Beast Mask | Max HP +12% | +28% CRIT DMG against DEF-reduced enemies; applying DEF reduction grants allies "Comburent" for +15% damage |
| Pioneer Diver of Dead Waters | +12% damage to debuffed enemies | +4% CRIT Rate; +8%/12% CRIT DMG vs enemies with 2/3 debuffs, doubled for 1 turn after applying a debuff |
| Longevous Disciple | Max HP +12% | +8% CRIT Rate (up to 2 stacks) when hit or when HP is consumed |
For planar ornaments, Bone Collection's Serene Demesne is the top choice. It adds 12% Max HP and grants 28% CRIT DMG once his Max HP reaches 5,000 or higher, which is easy for an HP-scaler. Duran, Dynasty of Running Wolves works well in Follow-Up teams, and Inert Salsotto is a clean CRIT-focused backup.
Best Light Cones for Mortenax Blade
His signature Light Cone, Reforged in Hellfire, is the clear best option. It increases Max HP by 30% and, once per wave, restores a fixed 20 Energy at the start of his turn. That Energy is the key piece. Without it, he struggles to land a turn-zero Ultimate and his Zone uptime drops sharply. The cone also applies "Purgatory" after a Skill attack, raising CRIT DMG taken by the target.
If you don't have the signature, Lies Dance on the Breeze is a strong 5-star alternative that boosts SPD and stacks DEF reduction. For a budget pick, Resolution Shines As Pearls of Sweat covers DEF shred at 4-star rarity.
| Light Cone | Rarity | Why it fits |
|---|---|---|
| Reforged in Hellfire | 5★ | +30% Max HP and fixed 20 Energy per wave for reliable Ultimate uptime |
| Lies Dance on the Breeze | 5★ | +18% SPD plus layered DEF reduction on enemies |
| Resolution Shines As Pearls of Sweat | 4★ | Cheap DEF shred via the "Ensnare" debuff |
Stat priority and trace order
Because his damage is built on Max HP and CRIT, you want crit substats and HP layered on top. Speed matters for keeping Charge and Zone rotations smooth.
| Slot | Main stat |
|---|---|
| Body | CRIT Rate or CRIT DMG |
| Feet | Speed, then HP% |
| Planar Sphere | HP% |
| Link Rope | HP%, then Energy Regen Rate |
| Substats | CRIT Rate / CRIT DMG, then HP%, then SPD |
For traces, upgrade his Ultimate first. It triggers Infinite Fury, plants the Zone, applies Balefire Bind, and unlocks the rest of his kit. His Skill comes next as his main damage source, followed by his Talent for the support side. His Basic Attack is last and offers the least value.
Best teams for Mortenax Blade
Mortenax Blade slots into almost any team that needs damage amplification, but he shines hardest next to DPS units that lean on their Ultimate or attack often. Every ally hit during his Zone feeds his Charge, so high action frequency directly translates into more of his free Skill casts.
Acheron is one of his strongest partners. His steady Balefire Bind application accelerates her Ultimate generation, and he fills her required Nihility slot. With a second Nihility unit present, his "Heart, Refined ad Infinitum" trace grants the team increased Ultimate damage, which stacks beautifully with her kit.
Ashveil and Feixiao are excellent Follow-Up and high-action DPS pairings, since their frequent attacks rush his Charge to 9 and trigger extra Skill damage. Tribbie is a standout support because she also scales with HP, raises the team's Max HP, and triggers Follow-Up attacks, lifting both herself and Blade.
| Team | Core idea |
|---|---|
| Mortenax Blade · Acheron · Cipher · Huohuo | Stack debuffs and Nihility synergy to fire Acheron's Ultimate repeatedly |
| Mortenax Blade · Ashveil · Tribbie · Hyacine | HP-scaling Follow-Up core with Hyacine raising everyone's Max HP |
| Mortenax Blade · Feixiao · Tribbie · Hyacine | High action frequency to spam his Charge-fueled extra Skills |
| Mortenax Blade · Acheron · Jiaoqiu · Hyacine | Heavy debuff stacking around Acheron's burst windows |
Sustain is flexible. Hyacine is the ideal healer because she boosts his Max HP, while Huohuo or Gallagher work when you want ATK buffs and Energy support instead. His Technique and traces also make him tanky enough that you can occasionally run him without a dedicated healer, leaning on his taunt and the 50% heal he gets when a killing blow lands in Infinite Fury.
Build him around HP, CRIT, and Energy, keep his Zone up, and let the team's attacks do the rest. Once the Charge loop is running, his free Follow-Up Skills and the constant Balefire Bind turn him into both a debuffer and a steady AoE damage dealer in the same slot.