What is a たる adjective, you ask?
Why, it’s an adjective that looks like a な adjective, but requires たる instead of な when it’s an adjective, and と instead of に when it’s an adverb.
They are not common generally, but there are definitely ones that exist in the top 15000 words in Japanese.
And some are taught on WK, but usually only when the たる form isn’t the only form. Some can also be used like regular adverbs or just plain nouns.
“Common” たる adjectives
堂々 (どうどう) - magnificent, grand, impressive, majestic (#5574)
依然 (いぜん) - still, yet, as it has been (#6362) (WK36)
漠然 (ばくぜん) - obscure, vague, equivocal (#8464) (WK33)
延々 (えんえん) - forever, endlessly, on and on, meandering, zigzagging (#8528)
冠 (かん) - best, peerless, first (#9918) (noun form taught on WK54)
断然 (だんぜん) - firmly, absolutely, flatly, by far, hands down (#10831)
断固 (だんこ) - firm, determined, resolute, conclusive (#11019)
公然 (こうぜん) - open, public, official, overt (#11194)
平然 (へいぜん) - calm, composed, cool, quiet (#13542)
確固 (かっこ) - firm, unshakeable, resolute (#14992)
Other たる adjectives that aren’t common but are taught on WK
整然 (せいぜん) - orderly, regular, systematic (WK13)
躍如 (やくじょ) - vivid, lifelike, graphic (WK47)
悠々 (ゆうゆう) - quiet, calm, leisurely, easy (WK54)
敢然 (かんぜん) - boldly (WK57)