This subclass has a great feel, and isn't nearly as overpowered as the old 3rd ed version could get. I'd propose a couple minor edits, but overall this is great raw material for use in a home game.

Specifically I'd specify "beasts" instead of "any creature", to prevent abuse of other critters that swarm by rules-lawyery players (true, all currently available swarms in the MM & VGtM are beasts, but that doesn't mean we won't see demon swarms or ooze swarms, etc in future products. Best to be pro-active with clarity). Also, I'd specify a little more clearly how "abiding by other beast shape restrictions" interacts with the "enhanced swarm" options (normally a lv 6 druid can't become a flying creature, but if you take the Winged enhancement, you get a flight speed). But these are all minor tweaks that are easily figure-out-able at the table with a good DM & a reasonable player.

The "blot out the sun" features are awesome, and feel powerful without being overpowered. And the utility of the defensive "One for Many, Many for One" options & other enhancement features make this feel like a viable alternative to the Moon Druid for folks who want dominance as a shifting druid. And "Countless Eyes" just feels cool... the kind of cool that I haven't felt since playing an Ananasi in 2nd ed World of Darkness back in the early 2000s. Great work!

Love the little details in this, like the artwork and the flavor given to some of the material components (Longevity is a particular favorite). As well-balanced as 5e spells can be, plus some awesome higher level stuff (Raise City?!?). Definitely fills a much-needed gap in spells for not just wizards, but also clerics, warlocks, & other casters.

Love that you converted all these 4E undead. Gives me so much more variety in my undead selections. Also keeps players on their toes, not knowing what kind of ghost/zombie/skeleton they're dealing with. Well-balanced also. Top marks.

Super useful. Varying threat levels to fill lots of gaps in your monster vault. Well-balanced foes for their given CRs, and nothing that feels too over-powered (unlike some of the MM foes). All-in-all an awesome product.