1) Booming Blade is a lot more dangerous than usual from these guys, because Potent Spellcasting adds your Wisdom on both the melee attack and the rider, and because Warcaster means that walking outside of the Cleric's threatened area to go punch someone else triggers the rider and then
another Booming Blade and then
another rider too. This works out to a significant chunk of damage at any level, especially considering it's not all of your action economy (you've still got your Concentration, your bonus action, and possibly minions).
While it doesn't "force" an enemy to stay still, it punishes them severely for not doing so. If they opt to walk over to someone with a couple less points of AC, the benefit of this is offset by living for less rounds and therefore being able to take fewer harmful actions. And staying still has considerable disadvantages of its own (like getting stuck in hazards, having Disadvantage on ranged attack rolls, being unable to adjust line of effect, etc).
In short, this Cleric has a damned fine OA and that is a very useful thing for a tank to have.
2) Spellbreaker is antimagic gold, and there's an awful lot of things you can do with it.
For example, casting Regenerate provides an hour long, Concentrationless buff that gradually heals over 600 hp throughout its long duration, helping to keep ally hp topped off between combats and picking up allies off the 0hp gate at the start of their turn without so much as a bonus action cost. But with Spellbreaker, it also removes a spell effect you don't like every round, again with no action cost from you. It’s like an hour long “every turn get a mini Healing Word / Dispel Magic at the start of your turn.”
For another example, Mass Healing Word is kinda like a bonus action Dispel Magic that targets your whole party and, as a bonus, also heals people a bit and picks them up off the 0hp gate.
For another example, Heal is an effective burst heal that also removes a variety of negative effects.
3) If you want even more antimagic, you can pick up Counterspell from the Azorius background from GGtR.
4) Warding Bond is a great 1 hour, Concentrationless buff that gives an ally +1 to AC and all saves (so, basically like a ring of protection. Or to put it another way, imagine if Shield of Faith exchanged 1 point of its AC bonus for +1 to all saves, then lasted 6 times as long and didn't take Concentration), and that's before it even does its primary job: Damage splitting. Also, unlike some other damage split mechanics in 5e, the damage you take on your end
can be mitigated by your own resistance (such as from Absorb Elements).
It's not the only good Concentrationless buff you have either. Freedom of Movement prevents all kinds of things for an hour, and Death Ward is an 8 hour one that gives the target an extra death gate. Sanctuary is a ranged bonus action that requires enemies to make a Wisdom save to target a creature at all and only costs a level 1 spell slot (and can interfere with action choices less than you might expect if you're attentive to action order and the like; you can use it at the end of your turn right after hitting someone, and simply maintaining spells you already cast like Spirit Guardians doesn’t end it). There's even stuff like, say, an upcast Continual Flame giving protection against magical darkness with an unlimited duration (since Darkness can't darken areas illuminated by a spell of 3rd level or higher, period). These things add up.
5) Minions! You can create and maintain a bunch of animated dead on your days off, who can then do things like spam grapples at people or just stand in front of allies to grant them cover against ranged attacks or block the movement of melee attackers, while firing ranged attacks of their own. They can also do things like administer potions to teammates, per the DMG rules. Don't forget that you can give general commands so that you don't have to use a bonus action every turn.
More situationally, you can also use things like Planar Ally and Planar Binding to contract bigger fish.
6) Spirit Guardians lasts 10 minutes, which is long enough for numerous encounters in a dungeoneering situation (meaning that it'll often already be up when a fight starts). SG not only creates a large zone of hurt that leaves your actions free on every round after you cast it, it also halves the speed of everything in that area without a save (and stacks with Difficult Terrain and the like). For melee foes, this can straight up prevent them from being able to reach your party. For ranged ones, it limits their firing lines (such as for firing around cover) and ability to kite.
7) Your lack of reliance on your Action to be threatening (thanks to powerful Concentration effects, bonus action tools like Spiritual Weapon, punishing reactions, and minions), you can totally do stuff like use the Dodge action if you need to, and still keep your DPR relevant. I've seen these guys just jump into a crowd of enemies with a Spirit Guardians they cast 3 encounters ago and use Dodge until they all melt (mook swarms tend to have a pretty hard time hitting a decent AC with Disadvantage). This is but one of many examples of how these guys are versatile and adaptable with their action economy.
As I put it in another thread:
Spoiler
Show
Quote:
Originally Posted by
LudicSavant
Quote:
Originally Posted by
KorvinStarmast
Our sixth level cleric waded into a pack of gnolls using Dodge action and Spirit Guardians. It was amazing how often that disadvantage thing caused a miss. Two rounds later, the remaining gnolls were trying their best to flee. (Needless to say, our other party members were also attacking, but that aura of angry spirits was very effective).
He had to make one con save, and made it easily.
Mathematically, a half-decent AC and Disadvantage makes it almost impossible for low-attack bonus enemies to hit you, which makes Spirit Guardians + Dodge a very effective way of taking out swarms.
For example, here's a gnoll's chance of hitting with Disadvantage at various ACs:
AC 19: 6.25% (1 in 16 attacks hit)
AC 21: 2.25% (1 in ~44.44 attacks hit)
AC 23: 0.25% (1 in 400 attacks hit)
8) You have tons of CC spells. For example, you can use Banishment to lock a dangerous target out of the fight while you clean up everyone else. Not to mention that Divine Word is like an AoE, bonus action, instantaneous Banishment against celestials, elementals, fey, and fiends (regardless of their hp) and also debilitates mooks or wounded foes with no save. Hold X is basically a death sentence if it lands (because of the autocrits). Command is a rare no-Concentration CC at low levels that can affect multiple targets if upcast. Arcane Abjuration and Turn Undead are effectively extra Concentrationless CC spell slots, too (up to 3/short rest).
9) Big Damn Wizard Spells. This part only comes online at level 17+ so I won't spend too much time on it, but you get a 6th, 7th, 8th, and 9th level Wizard spell known, and there are some golden choices on the Wizard list for your purposes, like Contingency or Foresight.
10) Your AoE damage isn't bad at all. Spirit Guardians just sort of passively melts things, but if that's not enough you've got options like Potent Spellcasting boosted Green-Flame Blade or Word of Radiance, or spells. Sure, stuff like Flame Strike and Fire Storm might not be the best AoEs around, but it's still enough to kill off swarms when stacked with Spirit Guardians and it's a heckuva lot more AoE potential than most martials get.
11) As long as you're up, actually taking allies out of the fight is hard. Even if enemies can get at your allies through all of your CC and pressure and buffs and such, allies at 0hp just pop right back up and many status effects just get cleansed. Even an ally who actually dies will get popped back up by Revivify while things like SG/SW/minions keep ticking away.
12) A high perception score at the front of the party's marching order helps to protect against Surprise, traps, and the like. Also has access to a variety of divinations and "information wars" abilities.
13) An important thing to understand is that you're never really doing just one of these things at a time. Your action economy is great. Just be careful to budget your resources properly for however many encounters a day you expect to fight (you should have enough for 6-8 encounter days if you're doing it right).