UnderRail Psi Guide — Schools, Innervation, and the HP Tradeoff

UnderRail is a single-character, turn-based CRPG, and psionics — psi — is the optional power system that, for a lot of players, ends up being the easiest strong way to play it. The reason is simple and worth stating up front: a psychic's entire toolbox of abilities is always available. There is no ammo to track, no reload, no weapon to swap. Every encounter becomes an ability rotation — read the room, pick the right tool, spend your psi points, repeat. That clarity is exactly what makes psi forgiving to learn even though, as you will see, the build asks you to play carefully.

This guide is a deep-dive into how psi actually works: how you unlock it and what it costs you, which stats drive it, the four schools, the psi-point economy, innervation, and how to survive as a deliberately fragile character. If you are still deciding what to play at all, the UnderRail build guide compares psi against the gun, crossbow, and melee archetypes.

What psi is, and why it is beginner-strong

A non-psi character lives and dies by a weapon skill, ammunition, and positioning. A psychic instead carries a set of innervated abilities that are always ready. Because the whole kit is on tap at any moment, you are rarely caught without an answer: a damage spell for a soft target, crowd control for a pack, a debuff for a heavy. Fights stop feeling like improvisation and start feeling like a rotation you can plan.

That is the upside. The honest counterweight is that psi is not free power — it is power you pay for in maximum health, in psi points, and in the discipline to focus. The rest of this guide is mostly about those costs, because understanding them is the difference between a psychic that steamrolls and one that runs dry mid-fight and dies.

The gateway — Psi Empathy and its permanent HP cost

Psi is not on by default. You unlock it by taking the Psi Empathy feat, which in the world means getting a pill from Dr. Pasquale at South Gate Station Medical to disable your psionic inhibitor. From that point you can learn and use psi abilities.

The catch is permanent and you should weigh it before committing the build: Psi Empathy imposes a lasting -20% maximum health. Later, the veteran feat Advanced Psi Empathy raises the total HP penalty to -45%. That is close to half your health bar gone, by design. A pure psychic is one of the more fragile characters in the game, and that fragility shapes every other decision you make.

Will and Intelligence — the two stats that drive psi

Two attributes carry a psi build, and they do different jobs.

• Will is your psi potency and a core driver of psi regeneration. It is the headline psychic stat — higher Will means your abilities hit harder and your active pool refills faster.
• Intelligence also feeds psi regeneration, and it does something Will does not: it caps how many abilities you can have innervated (prepared) at once. More on that limit below, but the short version is that Intelligence buys you breadth of toolkit.

You will not max both alongside everything else, so most psi builds prioritise Will for raw effect and invest enough Intelligence to comfortably innervate the abilities they actually want to use.

The four schools

Psi is divided into four schools, each with a distinct role. The first three are in the base game; Temporal Manipulation arrives with the Expedition DLC.

A useful way to read this table: Metathermics and Psychokinesis are your primary damage schools, Thought Control is your crowd-control and armor-ignoring school, and Temporal Manipulation is a support layer that buffs you and debuffs enemies rather than carrying a fight on its own. Most strong psychics pair one damage school with one control or support school.

Psi points, the reserve, and regeneration

Here is the economy that decides whether your rotation holds up over a long fight. Post-Expedition, a psi-user has 100 maximum Psi points in their active pool, plus a separate Psi reserve of roughly five times the active maximum that exists to refill the active pool.

Your active pool regenerates each turn. The regen formula is:

5 + (Will + Int) / 2, rounded down.

So a character with, say, 12 Will and 10 Intelligence regenerates 5 + (22 / 2) = 16 psi per turn. This is exactly why Will and Intelligence matter beyond raw potency — they keep your pool topped up between casts.

The reserve is the part that catches people out. Each time your active pool refills, it draws from the reserve. When the reserve empties, psi stops regenerating entirely until you refill it — and the only way to refill it is with psi inhalant consumables. Carry inhalants. Running the reserve dry in the middle of a long fight, with no inhalants on hand, is one of the most common ways a psi build stalls.

Innervation and the focus-one-or-two-schools rule

You cannot simply use every ability you know. You must innervate them first — effectively prepare a loadout of active abilities. The number you can innervate is capped:

2 + Intelligence / 2, up to a maximum of 8 (rounded down).

So Intelligence is what widens your toolbox, but even at the cap you are choosing 8 abilities, not 30. That alone forces selectivity.

The second, more important rule is the cost penalty for spreading across schools. Innervating abilities from multiple disciplines adds +10% base psi cost per extra discipline. Run two schools and there is a modest tax; run all four and every spell costs dramatically more psi. This is a deliberate nerf to "all-school" psi, and it exists to push you to focus one or two schools rather than dabble in everything.

The practical takeaway: pick a damage school and a control or support school, innervate the best abilities from those two, and leave the rest alone. A focused two-school psychic spends far less psi per turn and hits much harder than a four-school generalist who burns out their pool in two casts.

1. 1

   Choose your core two schools

   Pair one damage school (Metathermics or Psychokinesis) with one control or support school (Thought Control or Temporal Manipulation). Resist the urge to add a third.

2. 2

   Set your Intelligence for the loadout you want

   Innervation cap is 2 plus Intelligence over 2, max 8. Invest enough Intelligence to slot the abilities you actually use, and put the rest of your weight into Will.

3. 3

   Build a clean rotation

   Open with crowd control, follow with your damage school, and keep a panic button (a freeze, a fear, or a knockdown) innervated for emergencies.

4. 4

   Stock psi inhalants

   Your reserve is finite. Carry inhalants for long fights so an empty reserve never ends a rotation early.

Playing fragile — positioning and crowd control

Because Psi Empathy strips a large chunk of your maximum health, a pure psychic cannot afford to trade blows. The good news is that psi gives you exactly the tools to avoid trading at all.

Lead with control. Thought Control can lock down a pack with fear or rage and notably bypasses armor, which matters against the heavily armored enemies that shrug off conventional damage. Metathermics can freeze enemies in place while you burn them down. Psychokinesis can knock targets back and create space. Used well, you are deleting or disabling threats before they ever reach you.

Positioning does the rest. Fight in doorways and corridors so you only face one or two enemies at a time. Open from stealth or from range when you can. Keep an escape or a hard crowd-control ability innervated so a bad initiative roll does not end the run. With -20% HP — and -45% if you took Advanced Psi Empathy — a single bad turn in the open is far more dangerous for you than for almost any other build, so the discipline of never getting surrounded is not optional.

For character creation, the opening hours, and where psi fits into early progression, see the UnderRail beginner guide.

Bringing it together

Psi earns its reputation as the easiest strong archetype because the toolbox is always in your hand — but that reputation hides three real costs you have to respect. You pay in permanent health for the Psi Empathy feat, you pay in psi points and a finite reserve that only inhalants refill, and you pay a focus tax that punishes spreading across all four schools. Honor all three — focus one or two schools, manage your pool and reserve, and win through crowd control and positioning rather than durability — and psi becomes one of the most reliable and satisfying ways to play UnderRail.