If you center the Cloudkill spell on yourself, does the fog expand outward from you on subsequent turns?

Question

The description of the cloudkill spell says (emphasis mine):

You create a 20-­foot-­radius sphere of poisonous, yellow‑green fog centered on a point you choose within range. The fog spreads around corners. It lasts for the duration or until strong wind disperses the fog, ending the spell. Its area is heavily obscured.

[...]

The fog moves 10 feet away from you at the start of each of your turns, rolling along the surface of the ground. The vapors, being heavier than air, sink to the lowest level of the land, even pouring down openings.

If you center the cloudkill spell on yourself, does the cloud expand outward on subsequent turns, since it moves "away from you"? Or would it stay as a single unit and roll off in a direction of your choice?

Answer 1

"Moves", not "Expands"

This is a circumstance the rules don't cover perfectly (presumably because the designers didn't consider a character being stupid enough to cast this on themselves) so officially you will need to ask your DM, but here is how I would rule it.

The important part of the spell's text is highlighted below:

The fog moves 10 feet away from you at the start of each of your turns [...]

The key word here is "moves", implying that the entire cloud moves together. The direction it moves in is somewhat arbitrary. Either you can decide your your DM will decide for you.

Allowing it to expand increases the power of the spell

To rule the other way and rule that the cloud can expand outwards in a ring from its point of origin makes it a more powerful spell. I'm not going to get into the math, but if the cloud expands outward each round, it also increases in area. Increasing the area of effect of a spell increases its power level.

Answer 2

You cannot center it on yourself

At least, not if you play on a grid. You, a presumably medium-sized caster, occupy a 5'x5' square, and are assumed to be entirely within that square (see, for example, the rules on cover, DMG 250, 251).

Then, as directed by the spell, "You create a 20-foot-radius sphere of poisonous, yellow-green fog centered on a point you choose within range." However, the "point you choose" must adhere to the DMG rules about areas of effect (ibid):

Choose an intersection of squares or hexes as the point of origin of an area of effect, then follow its rules as normal.

Because you are within the square, but the point of origin of the spell is at an intersection of squares, the closest you can get the origin of the spell to yourself is one of the intersections of the square you are in, which is not quite centered on yourself, but instead is touching your outer boundary.

See, for example, On a self-targeted spell like Spirit Guardians, can the caster change the intersection corner on a grid?

Then, at the start of each of your turns, the cloud moves away from you. If you haven't moved since casting it, on the next turn it will "remain as a single unit" and move away from you in the direction of the diagonal of the square you are in, from your center to the intersection you selected as its origin.