CIRCLE OF WILDFIRE
Druids with Fireball!
Relentless Naming shouldn’t do force damage in my opinion.
Druid's with Fireball was my first thought too! Love it.
I kind of also agree with you on the Relentless naming thing. I think it was an effort to ensure that it had some effect. There are many things out there that grant resistance to Psychic damage. Nothing does that really for Force.
Twilight cleric's Channel Divinity power is extremely powerful. For a fun minute (so, basically an entire encounter), everyone within 30 feet of you gains a d8 temp HP every round? WOW. That's HUGE. That stands people up from negative HP, doesn't it? But, the fact that every single round, every single PC is getting bonus HP is a big, big advantage. Plus, at the end, everyone is going to have a d8 temp HP to take into their next encounter.
Recharges on a short rest? That's a big deal.
It helps give Cleric back some of that healing power that they've been losing to Druids steadily in 5e outside of the Life domain, that's for sure.
Balance wise though? I mean it's 4.5 hp per round on average. Depending on what you're fighting, that might barely make a difference in a combat.
It is situationaly bad because it imposes dim-light on your party as well as enemies. So many people depending on party mix will be at disadvantage on sight-based Perception checks.
It kind of undercuts the importance and secret-ness of a true name if a can be sucked out of your head by a level 2 wizard.
Drakoth, Demon Lord: "I failed a DC 13 save, now the most secret aspect of my very nature and the one true path to ultimate power over me is known by a guy with 8 hp"
I'm all for words of power and naming I guess, but I feel like this either really cheapens the meaning of a true name, or else it REALLY makes it too easy to learn the most import secret of someone's existence. (Is Onomancy a commentary on Identity Theft in the information age?)
It also has some implications for people in disguise, shapeshifting, possessions by ghosts, etc.
I mean... I guess maybe it's possible for some high CR fiends to fail that check, but all the Demon lords I looked at have both legendary resistances AND the lowest Wis save I saw was +10. So... they probably aren't going to fail it
Even at the top end with max stats, etc. most of the Demon lords would only need an 8 or higher to make the save.
It would just give them another reason to hate and hunt the party too. Fun RP
I love the implications it has on the disguise/shifting/etc portion of things. Every Royal should have an Onomancer on their staff at the front door for important meetings and such.
It's also pretty limited 1-5 times per long rest isn't that many, and it burns a bonus action. If something has Legendary Resistances, it might be a good way to burn through some of those before casting other more powerful spells.