Can an incorporeal enter inside a corporeal to hit them from the inside?

Question

This is a tactic that the rogue in our party, becoming incorporeal (perfect Wight) has used in the past to kill creatures bigger than her size, avoiding being hit or spotted by them. Also this was used to avoid difficulties in causing sneak attacks to enormous or gargantuan creatures.

Is this possible?

When entering in the corporeals the player argues that there would not be any attack of opportunity before her ghost touch attack to enter as she was hiding in plain sight and was not noticed by the enemy.

Is this correct by the rules?

Answer 1

Yes

While considered by some not to be an authoritative source, the Rules Compendium has more and newer rules on incorporeality. Page 65 this to say regarding incorporeal creatures sharing corporeal creatures' space (emphasis mine):

An incorporeal creature can occupy the same space as a corporeal creature, unless the corporeal creature is entirely surrounded by a force effect. An incorporeal creature entering a corporeal creature’s space (or vice versa) provokes attacks of opportunity as normal for moving into another creature’s space. The entering creature must then succeed on a touch attack against the target to share the same physical space. If the target is helpless or doesn’t resist, no attack is necessary. If the attack succeeds, the entering creature moves into the target’s space. This attack deals no damage, even if the entering creature’s touch attack normally deals damage. If the attack fails, the entering creature returns to the space it occupied before entering the target’s space.

An incorporeal creature occupying the space of a corporeal creature might gain cover, while the corporeal creature might gain concealment. See the Sharing Spaces table. For example, a shadow sharing the space of an ogre gains cover, but the ogre gains no benefit. A shadow sharing the space of a halfling grants the halfling concealment, but gains no benefit itself. A shadow sharing the space of a human gains cover, and the human gains concealment. This cover or concealment affects even attacks made by the other creature sharing the space.

Either creature can end the “sharing” of a space simply by leaving that space. Doing so leaves the other creature in the space. If the leaving creature moves more than 5 feet, this movement provokes attacks of opportunity as normal for leaving a threatened square.

Note that if the corporeal creature is the same size or smaller than the incorporeal creature, it has concealment from the incorporeal creature. Unless your Epic rogue can ignore concealment, or is only using this against foes larger than her, she may not be able to sneak attack them. If the corporeal creature is the same size as her, then it can attack her but she has Cover (PH, 150), gaining +4 to AC. If the corporeal creature is two or more sizes larger than her, it can't target her since she'd have Total Cover (152).

If she was effectively concealed from the creature whose space she intended to share prior to attempting to share the space, then your Epic rogue seems to be correct regarding the attack of opportunity. So it appears she seems to be playing this correctly, though the creature whose space she's sharing can potentially exit the space.

If it survives her attacks.