So I hemmed and hawed on whether to buy this, or the Lyre of Progression, or the Rogue's Cowl. Made it past the tricky entrance to level 3, with only one death. Went back to town and bought the Horn.

Anyhow, I read that your summons will stick with you until death, unless you leave them behind. So, always keep a character near. I was moving outward here on level 3, and had my 3 ranged characters standing right next to my berserkers. My Thief went to get loot from an skellie archer.....couldn't fit it all, so sent my leader......when leader starts to return, poof, all my berserkers disappeared.

WTH? So, now I'll have to start this again, or be without them until rest. Was that wrong, how it works? Why did they disappear?

There are no summons which stay forever. All disappear after some time.

It's still a damned useful item. The summoned berserkers are a lot stronger than most summoned creatures, and they still last a fair bit of time.