This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
煋 combines 火 (fire) with 星 (star). The exact historical development is uncertain, but it likely represents a bright, star-like fire or a specific ancient name.
The left side is fire 火 and the right side is star 星. Picture a fire burning so brightly that it looks like a star, but remember that the actual meaning is unclear.
For セイ, imagine a star (星) shining with a pure, say-like light: say -> セイ, but the fire makes it uncertain.