You attempt to beguile a beast that you can see within range. It must succeed on a Wisdom saving throw or be charmed by you for the duration. If you or creatures that are friendly to you are fighting it, it has advantage on the saving throw.
While the beast is charmed, you have a telepathic link with it as long as the two of you are on the same plane of existence. You can use this telepathic link to issue commands to the creature while you are conscious (no action required), which it does its best to obey. You can specify a simple and general course of action, such as "Attack that creature," "Run over there," or "Fetch that object." If the creature completes the order and doesn't receive further direction from you, it defends and preserves itself to the best of its ability.
You can use your action to take total and precise control of the target. Until the end of your next turn, the creature takes only the actions you choose, and doesn't do anything that you don't allow it to do. During this time, you can also cause the creature to use a reaction, but this requires you to use your own reaction as well.
Each time the target takes damage, it makes a new Wisdom saving throw against the spell. If the saving throw succeeds, the spell ends.
At Higher Levels. When you cast this spell with a 5th-level spell slot, the duration is concentration, up to 10 minutes. When you use a 6th-level spell slot, the duration is concentration, up to 1 hour. When you use a spell slot of 7th level or higher, the duration is concentration, up to 8 hours.
I notice it doesn't say you can't order the target to do something obviously life-threatening.
I believe that’s intentional
It does only work on beasts though, so if there's a Totemic person in your group (Totem Barbarian), or just a person that doesn't agree with animal cruelty; you probably shouldn't.
Is this a potential counter to Circle of the Moon druids?
Could this spell be used to dominate a shambling mound? Or does this spell only work on creatures with “beast” in the description?
They need to have the monster type "beast"
A shambling mound is a plant so this wouldn't work.
No (shambling mound is a plant, not a beast) but you do have dominate monster that works on any creature
Can a player use this spell to control someone who has polymorphed into a beast?
They’re a beast when polymorphed, so yes! I think the main idea of this spell is to be a counter to polymorph, considering how rare beasts are at 7th level.
It could be, sure. Your type is "beast" while in the form of a beast.
can this spell be used against a Familiar?
No, familiars specifically are not beasts, they are celestials, fiends, fey, or occasionally undead, so this spell would not work on them.
If a druid turns into a beast, does this turn their creature type into Beast?
If so, can you cast this spell on them?
If so, does it end if they turn back into their original form?
Furthermore, if you are a Mark of Handling Human spellcaster, can you cast this on your Thri-kreen party member?
Furthermore, if you are a Mark of Handling Human spellcaster, can you cast this on a Tarrasque?
I bet House Vadalis has a Tarrasque.
I wouldn't call it a counter to mooncircle druids.
Yes, technically the spell works against druids in beast form.
BUT you need to remember (or just be made aware of the fact) that druids keep their mental capacity stats (WIS, INT, CHA) and their saving throw proficiencies in beast form.
And since Wisdom is a druid's main stat and they are proficient in WIS saving throws, you will have a hard to time to even land this spell on a druid. Even more so to keep it up once he/she starts taking damage.