I Maxed Mortenax Blade — Here's How to Actually Build Him

Mortenax Blade dropped in 4.3 as a 5★ Fire Nihility unit, and his kit is genuinely unusual for the path. Most Nihility characters just sit there and debuff. He debuffs and deals real damage — a hybrid that's part sub-DPS, part support, built around burning his own HP.

I leveled him to 90, kitted him out, and ran him for a while. Here's how to build him, who to pair him with, and whether his Eidolons are worth chasing.

How he actually works

The one-line version: he spends his own HP to deal damage, and his Ultimate opens a field that drops him into "Boundless Wrath."

His Skill is the main damage source, and it scales off his Max HP while consuming a chunk of his health — which is why HP is his single most important offensive stat, unlike most Nihility units. Cast the Ultimate and you enter Boundless Wrath: the field deploys, and the full kit unlocks — crit bonus, an enhanced Skill, and energy generation whenever allies hit weakened enemies.

While the field is up, he applies Pyronexus (def shred + vulnerability) to enemies. Here's the key bit: during the field, anyone on the team hitting a Pyronexus'd enemy regenerates his charge. Hit 9 charge and he automatically fires off an extra Skill — counted as a follow-up attack, with no HP cost.

So he naturally slots into two kinds of teams: high-frequency attackers and follow-up attack comps. The more your team swings, the more bonus Skills he triggers, and the smoother his whole rotation runs.

Trace priority

Ultimate goes first, no debate — it's the switch that turns on Boundless Wrath, deploys the field, applies Pyronexus, and unlocks his core loop. Most of his damage and team support hinges on keeping that state up. Skill second, since it's his primary damage. Talent third for the support. Basic attack you can flat-out ignore until everything else is done.

Best light cones

Top pick: Reforged in Hellfire (5★, signature)

Boosts Max HP, which feeds straight into his damage. More importantly — it restores 20 energy at the start of his turn, once per wave. That energy restore is critical: without it, he can't fire his Ultimate on turn 0, and his Ultimate uptime drops hard. The gap between having his signature and not having it is mostly about how smooth his rotation feels.

5★ alternative: A Trail of Bygone Lies (the "lies on the wind" cone) — gives +18% SPD and applies def-down on attack. Solid choice.

4★ budget option: Resolution Shines As Pearls of Sweat — if you want something easy to get, this def-shred 4★ does the job.

Relics & ornaments

Relic set

#1: Smith of Sky-Splitting Tools (Inquiry of the Heaven-Forging Smith)

This set is built for him — it hands over the HP% and CRIT DMG he wants, and the 4-piece gives the whole team a 15% damage boost. He already runs on def-shred, so he triggers the team buff perfectly. Farm this, no second-guessing.

#2: Pioneer Diver of Dead Waters — damage doesn't drop too much, but the team loses the Smith set's party-wide buff, so it's the weaker option overall. Fine as a transition before you farm the Smith set.

Planar ornaments

Stat priority

Main stats: Body → CRIT Rate / CRIT DMG, Feet → SPD > HP%, Planar Sphere → HP%, Link Rope → HP% > Energy Regen%. Substats: CRIT Rate = CRIT DMG > HP% > SPD.

Team recommendations

He shines in teams that can eat his debuffs, generate his charge, and make use of follow-up attacks. A few that work:

Acheron team — one of his best pairings. His steady debuff application speeds up Acheron's Ultimate charging, and while his field is active, the team keeps stacking Pyronexus to feed her stacks more reliably. He also satisfies her need for Nihility teammates, making him an excellent support slot for her.

Follow-up comps (Phainon / Feixiao carry) — Phainon's high attack frequency starts Mortenax Blade's charge system faster; Feixiao also benefits from his debuffs and high action frequency. Both comps lean on frequent attacks, so they reliably trigger his bonus Skills, which means more damage and a cleaner loop. Worth noting: the Phainon version still lacks a sustain slot that synergizes with both units — this comp has real upside once a future main DPS and dedicated sustain fill it out.

Are his Eidolons worth it?

E1 and E6 are the two breakpoints. E1 gives team-wide RES PEN and better Ultimate uptime for great value; E6 sends his damage through the roof. The middle Eidolons (E3/E4/E5) are mediocre — don't chase them specifically. If you like the character and want to invest, stop at E1, then save up and push E6 when you can. That's the ideal pacing.

Is he worth building?

Mortenax Blade is shaping up to be one of the most flexible units active in HSR right now — def shred, vulnerability, and a charge system that fires off bonus Skills, making him valuable in both debuff-centric and follow-up comps. He's not a traditional Nihility unit that only weakens enemies; he supports the team while dealing steady damage himself.

If you're running Acheron, Phainon, or Feixiao, he's a real upgrade worth pulling and building. And if you want to push his Eidolons, HSR uses UID top-up — just your UID, no password, delivered in 1-3 minutes. Much safer than the account-and-password top-up model some games use.

Kardz runs a 6-11% standard discount on Stellar Jades and Oneiric Shards (exact tier pricing on the product page), with a first-order discount stacking on top. If you're planning a big Eidolon push, that first-order window saves a meaningful chunk — enough for an extra 15-20 pulls.

→ Kardz HSR top-up page

One honest note though: E0 Mortenax Blade already pulls his weight in the right team. Eidolons are the cherry on top, not the requirement. Check whether you've actually got the Acheron/Phainon pieces first, then decide how much to invest.