Phasing
Phasing is the ability to walk through walls. The property is possessed by xorns, earth elementals, ghosts and shades[1]. This does not allow one to cross pools of water, or walk through those rare areas that are designated as non-phasing: all of Sokoban, an almost-dividing wall in Moloch's Sanctum,[2] the inner wall around the Wizard's Tower, and the entire Astral Plane. SLASH'EM introduces the spellbook of passwall, whose corresponding spell will temporarily grant phasing. UnNethack introduces the Iron Ball of Liberation (the convict quest artifact), which grants temporary phasing upon invocation.
Strategy
The use of polymorphing into a phasing monster to gain a combat advantage is dubious, as neither the xorn nor the earth elemental are particularly tough, and cannot wear body armor or cloaks (in fact the earth elemental cannot wear anything).
Warning: When morphing into a xorn any worn armor or cloaks will likely be destroyed, so remove these items before making the transformation.
Phasing may be used as an alternative to digging, or to access items embedded in walls or to retreat if you're getting surrounded.
Strategically, phasing can be used to quickly bypass some areas of some maps. It is one way a rogue character can solve their final quest puzzle. Some players will polymorph into a phasing creature for Gehennom mapping to make the search pattern simpler. Walking on the walls helps avoid traps.
Speed runners strive to phase into the correct turret of the Castle, and from there to the gem chamber (for Luck) and the trapdoor to the Valley of the Dead. The Valley, Asmodeus' Lair, and Vlad's Tower can be noticeably abbreviated with phasing (unless you want the loot). However, phasing is not an all-round solution to non-teleport mazes because a xorn is a slow monster and the lack of a cloak makes it dangerous to fight level draining monsters, shades, or Yeenoghu.
Approaching a shopkeeper across an unmapped level and stealing from the shop can be other situational uses of phasing. However, other polyforms are faster and better fighters.
References
This page may need to be updated for the current version of NetHack.
It may contain text specific to NetHack 3.4.3. Information on this page may be out of date.
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