This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
焫 combines 火 (fire) with 芮, but the historical development and original meaning are uncertain.
The left side is fire 火, and the right side 芮 looks like a plant inside an enclosure. Picture a rare, mysterious fire burning inside a hidden garden.
For ゼツ, imagine a rare fire that burns with a sharp, zets-like hiss: zets -> ゼツ, the sound of the strange flame.