A friend and I are having a dispute on the wording of the spell description and mechanics behind Dimension Door. He attempted (and succeeded) in a creative use of the spell in a game he is in, and while I fully support using the tactic via Rule of Cool, I had doubts of the "legality" of the use in a stricter sense. In full, as a means to rescue an ally from a rough situation, he used Dimension Door to teleport onto his friend's shoulders, and the next turn used a second casting of Dimension Door to teleport the both of them away to safety.
The spell description for Dimension Door states "…You arrive at exactly the spot desired…" (Player's Handbook, pg 233. Emphasis mine). Previous questions have already established that similar spells (Misty Step) can be used to teleport into open air (here and here), but would that allow teleporting onto the shoulders of an ally (non-hostile creature)?
The Dungeon Master's Guide gives rules for climbing onto larger creatures (pg 271). This is discussed in context of teleportation spells in this thread. This, however, never addressed the issue of teleporting onto a creature, merely using teleportation magic to get to that location and the checks/saves required to stay there. (I emphasize 'onto' here to distinguish from the workaround of teleporting slightly above the target creature and falling onto them. The example I'm seeking is full-on piggy-backing.)
The bigger issue, in any case, is what constitutes an occupied space, for the context of the spell. The later half of the spell description for Dimension Door states:
"…If you would arrive in a place already occupied by an object or a
creature, you and any creature traveling with you each take 4d6 force
damage, and the spell fails to teleport you."
My friend is adamant that as long as there is no other creature riding on the shoulders of the target ally, the space is unoccupied. There is nothing physically in the three dimensional space above the other creature. However, as per the Player's Handbook rules for a creature's space,
"A creature’s space is the area in feet that it effectively controls
in combat, not an expression of its physical dimensions" (pg 191).
To me, this insinuates that while the space above the creature's shoulders might not be occupied in the same sense as teleporting into a solid wall or inside of another creature would be, it still counts as occupied for the purpose of spells, abilities, and actions.
The closest definition for occupied space that I could find in other answers was included in this one. That being, a space is occupied when there is enough of something in that space to make it difficult for a creature to stay there. Rules on moving around other creatures clearly states that:
"you can’t willingly end your move in its [a creature's] space"
(Player's Handbook, pg 191).
The two ways I could see around that would be claiming the target creature to be the spell caster's "mount", or through a grapple check.
"A willing creature that is at least one size larger than you and that
has an appropriate anatomy can serve as a mount, using the following
rules" (Player's Handbook, pg 198)."When you want to grab a creature or wrestle with it, you can use the
Attack action to make a Special melee Attack, a grapple" (Player's
Handbook, pg 195).
The issue with these mostly deal with size difference and action economy. To be a mount, the target creature would need to be at least one size category larger (and presumably quadrupedal, but let's ignore that for the immediate moment). For the grapple check, either the spell caster would need to have cast Dimension Door as a bonus action using Quicken Spell, or the target creature would need to use a readied action or reaction to initiate the grapple. While the wording for the grappling rules don't specify that the creatures involved occupy the same space, it seems likely to me. Though, even if successful, the grapple would make it difficult to follow up with a second spell without first breaking the grapple (and moving to separate spaces).
I apologize for rambling on, and I'm sure I've already answered my own questions, but I'd like confirmation from the rules on just what constitutes an "occupied space", and whether someone can Dimension Door onto the shoulders of another creature?
Best Answer
Yes, but you cannot stay
Dimension Door's limitation does not talk about space, it talks about place
So the limitation does not prevent you from teleporting into another creature's space, only into the place filled by their physical body. (For an in-depth discussion how hard it may be avoid that with a moving target, see Jack's answer.)
However, the rules about space prohibit you to move through (or enter) a same-sized hostile creature's space (PHB, p. 191):
So, even though the spell would allow ending up on an enemies shoulders, the general rules for combat do not allow this. The caster can teleport onto an allied creature's shoulders, but will not be able to stay there:
(There is a clarification that you cannot end any part of your move in another creatures space, but I think that only refers to your move action, which is not being used here).
Whether your DM allows two characters to stay in the same space when one rides on the other's shoulders is independent of dimension door, and up to DM ruling.