flame, blaze, glowing, blazing
焰 centers on a visible, intense fire: a flame, blaze, or something glowing and blazing. It describes the bright, hot, leaping part of a fire.
焰 combines 火 (fire) with 臽, which likely contributes the sound and may relate to a pit or trap, but the exact historical development of the compound is uncertain.
The left side is 火 (fire), and the right side 臽 looks like a pit with something inside. Picture a pit of fire with flames leaping out: 焰 is a blazing flame.
For エン, imagine the flame of a candle at the end of a long hall: end -> エン, and the flame glows at the end.