I have a cleric who wants to ride a war horse (movement speed 60′, dash speed 120′) with a spell cast on himself. Controlling a mount does not appear to cost an action, you simply decide if you are controlling it or not.
The 5th level cleric casts Spirit Guardians to create a 15′ radius, 3d8 kill zone centred on himself. He controls the mount to dash along the front ranks of the orcs staying just out of reach and placing the front two rows into the area of effect.
Riding the 120 feet he forces up to 48 orcs to enter the area of effect (admittedly against their will). If he can hold his concentration he keeps this up for 10 minutes (100 rounds). I’m fairly sure once the orcs start pelting him, his concentration will break but gosh what a first round.
Am I correctly interpreting the way the spell works?
Since this question was asked, Jeremy Crawford has changed his mind on how this works.
In the April Rules Answers column for Sage Advice, Crawford has this to say:
Does moonbeam [or Spirit Guardians] deal damage when you cast it? What about when its effect moves onto a creature? The answer to both questions is no.
He goes on to elaborate:
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.
This is the list of spells the article applies to:
blade barrier
cloudkill
cloud of daggers
Evard’s black tentacles
forbiddance
moonbeam
sleet storm
spirit guardians
Contrast with his original tweet, and follow-up.