Pentaxis, the Guardian of the Library
Boss · Chapter 2: The Seed
Pentaxis guards the Library's lower floor with 2,572 HP and the widest toolkit of the chapter 2 bosses. Nothing it does is subtle, but it does a lot of it at once: fireballs, zigzag shots, chakrams, lasers, spin dashes, and sequenced floor explosions.
The fight is projectile management more than dueling. Most of its tools come with a marker (a crosshair for the laser, ordered flashes for the explosion chains, fixed zigzag lanes), so deaths here are almost always to the pattern you stopped watching.
Aimed fireballs, some bursting into area versions where they land.
Projectiles that bounce along fixed zigzag lanes on the arena's flanks. The lanes never change; stay off them when the volley starts.
Thrown blades that travel and return, cutting through positions that were safe a second earlier.
A laser preceded by a crosshair marker tracking your position. Break line of movement just before it locks.
It spins across the arena and sprays bullets when it hits a wall, so the dodge has two parts: the dash, then the splash.
Floor explosions detonate in sequence across the room. Read the order and walk the pattern; running randomly is what kills.
A jump onto your position that pushes a shockwave ring outward on landing.
It hides and reappears elsewhere to reset the geometry of the fight.
In hard mode every boss also draws from the pool of 10 modifiers listed on the bosses page, from flat HP and defense gains to elemental resists and ignoring your defense or evasion.
Defeating Pentaxis for the first time adds these to the item pool: