These are "unique drops". Most of the old base game dungeons have them. Even some of the older DLC dungeons, like White Gold Tower, had them. Where set items dropped and what set items dropped were very random and sporadic back in the day. It was fairly common to spend hours running White Gold Tower and not get a single item set piece to drop! It was maybe 50/50 if you got a set piece or random vendor trash from a boss. If you had a bad run, you might get nothing but vendor trash from every boss. And no ability to trade with group mates either!

The "unique drops" were special named items that only dropped from a specific boss. So if you were trying to farm in the old days, you might just rush to the boss with a "unique drop" and hope to get it. Unique drops were loot more often than regular set items. The opposite of now! Where every boss is a guaranteed a generic set item drop and the named unique drops are still random and sporadic, like all gear was back in the beginning.

tl;dr = old special random drops from certain bosses that have been around since game came out. Kind of obsolete in 2023 with way gear drops are more regular and curated.