The idea is this - at any time at least one enemy is on screen (or within attack range rather, to account for close up camera vs distant), show a “thread indicator”, for example up at the top. This will be a circular progress bar, 100% filled with red initially, and going down to 0%, filled with black, as you slain the monsters. In the center it will show the highest surviving monster level and hero/boss monster color if applicable. The usual “relative to character’s level” color scheme can apply. The indicator can reset back to “100% filled” state as soon as new monsters appear in range, to simplify coding effort (but not if they are summons).
Why is it useful? Helps to see if there is a hero monster on sight. I play close up, so I get better visuals and more fun from game. This results in some ranged hero monsters often one-shotting me (caster build), just because I didn’t see them soon enough. Another case is when a hero monster is behind a wall or similar. You don’t see them, all monsters gone, and then suddenly this guy sends you back to base (especially true for strong, but slow moving enemies). And yet one more use case - if you play MP, and you encounter boss monsters, would help to see they are actually 20 lvls below you, so no thread really.