-It can move, and it causes damage at the start of the turn or when a creature enters it. This means that the cleric can just move around and hit anything with it, the bigger it is the more it can hit, and the easier it is to stay away from opportunity attacks as they do. (this is how it works according to sage advice.)
No it isn't. Sage advice specifically says. "Creating the area of effect on the creature or moving it onto the creature doesn’t count.". The cleric can move so the creature will be subject to damage when its turn comes up, but he can't run around tagging a bunch of people.
Specifically talking about this spell, here it is https://www.sageadvice.eu/2015/03/16/spirit-guardian/ So yeah, if you ever need good evidence to show your players that sometimes sage advice should be ignored, this is a great exhibit A.
Page 18 of sage advice compendium also specifically mentions spirit guardians and contradicts that.
Here are some spells with the same timing as moonbeamfor their areas of effect:blade barrier cloudkill cloud of daggers Evard’s black tentacles forbiddance moonbeam sleet storm spirit guardians. Reading the description of any of those spells, you might wonder whether a creature is considered to be entering the spell’s area of effect if the area is created on the creature’s space. And if the area of effect can be moved—as the beam of moonbeam can—does moving it into a creature’s space count as the creature entering the area? Our design intent for such spells is this: a creature enters the area of effect when the creature passes into it. Creating the area of effect on the creature or moving it onto the creature doesn’t count. If the creature is still in the area at the start of its turn, it is subjected to the area’s effect
I rule that if using a grid then everything is squares. No center points, no radii, no diameters, just squares. No fractional squares either, just squares.
For example, sword burst affects everyone 5ft around you, so on the grid it affects all 8 squares around a medium creature (and all 12 squares around a large creature).
For example, fireball is a 20ft radius sphere, so on the grid it affects all targets in a 40ft×40ft square (square for simplicity and ease of use).
Players of spellcasters don't get to place centers or argue about intersections of grid lines - they just get to draw around a pattern of squares.
Page 18 of sage advice compendium also specifically mentions spirit guardians and contradicts that.
I rule that if using a grid then everything is squares. No center points, no radii, no diameters, just squares. No fractional squares either, just squares.
For example, sword burst affects everyone 5ft around you, so on the grid it affects all 8 squares around a medium creature (and all 12 squares around a large creature).
For example, fireball is a 20ft radius sphere, so on the grid it affects all targets in a 40ft×40ft square (square for simplicity and ease of use).
Players of spellcasters don't get to place centers or argue about intersections of grid lines - they just get to draw around a pattern of squares.