It's there (at the end of this post), but I won't debate that one with you. The rules evidence is there to support this, but not strong.
Would this mean that a blind target would not get a save when being forced into hindering terrain? What if the illusion was that the floor was solid, when in fact there was an acid pit beneath it?
The answer is that both squares are hindering terrain.
The Blind creature gets it because it is actually hindering terrain, even if he does not know it.
The unblind creature gets it because it is actually hindering terrain. Lightning firing into those squares is no different than the DMG example of fire in a square.
Cite a quotation that a square that causes damage when entered is not unsafe. Cite a quotation that a square must do constant damage in order to be unsafe. Cite a quotation that the unsafe square has to be a zone or an aura.
You won't find such rules.
Is it a safe square? No. Will a creature take immediate damage when it enters the square? Yes. Hence, it is unsafe. DMG requirement for an unsafe square satisfied.
Does Rain of Steel create a ring of Hindering Terrain around the fighter?
Irrelevant to the discussion. The square is safe at the point in time that the creature is slid into it. It is only unsafe when the creature starts his turn within it. An ally of the creature can move it out and it's totally safe. The encounter can end, the PCs all die before the creature's turn and it's totally safe. It's a conditional danger, not definitively unsafe.
Are you claiming that the adjacent squares to the Storm Pillar are safe squares to enter???
Using forced movement to pull, push, or slide a creature onto ice, or into a pit, or into a cloud of daggers is a clever tactic.
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Hindering terrain prevents movement (or severely punishes it) or damages creatures that enter it, but allows line of sight.
Cloud of Dagger is not Difficult Terrain, Blocking Terrain, or Challenging Terrain, so by definition, it must be Hindering Terrain if it is used as an example in the section on forced movement through terrain.
Cloud of Daggers is an effect that causes damage if the creature enters the square. Storm Pillar is an effect that causes damage if the creature enters the square.
They are both unsafe. Both cause immediate damage.
I really cannot understand how an effect that causes damage when a creature enters a square cannot be considered an unsafe square and hence hindering terrain.
Note: the "allows line of sight" reference supports my contention that the
orginal intent was for creatures to know about the danger to react to it. However, it is worded really badly. A blind man would still be capable of saving over a normal pit (he shouldn't, he doesn't have line of sight, but the square allows it), but a pit in a zone of darkness would not be hindering terrain because creature cannot see it (i.e. no line of sight in the square). Unfortunately, they did not word it properly. But, the original intent can be gleaned. However, I won't debate that. I'll let you win the "it's in the mind of the creature" debate. But, you don't win the actual debate. Storm Pillar squares are definitely unsafe squares, no different than Cloud of Daggers, Lava, or a Wall of Fire.