prairie fire
燹 centers on a large, uncontrolled fire spreading across open grassland: a prairie fire.
燹 combines 火 (fire) with 豩, which likely contributes sound and possibly the idea of wildness or spreading. The exact historical development is uncertain.
The top component 豩 looks like two wild boars (豕) side by side, and the bottom is fire 火. Imagine a pair of boars stampeding through dry grass, and the friction of their hooves ignites a massive prairie fire.
For セン, picture a thousand (千, せん) acres of prairie ablaze: 千 -> セン, and the fire spreads across the vast grassland.