Just killed him today and had two curious things occur: he ran at 50% even though he was a red con to our group, and he dropped trash - an Onyx Earring (no stats), and a Blackburrow Gnoll Pelt.
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Jiriki Saonserei, 110/35k AA Wood Elf Ranger Anadriel Starfire, 107/18k AA High Elf Mage Odran Kegslayer, 98/12k AA Dwarf Berserker
a gnol commander, an elite gnoll guard and a gnol arcanist CAN see invis...however for the commander and guard it's rare (usually like 1 out of every 25 mobs will see invis) as for the arcanists it seems half of them do and half of them dont. other than undead Tranixx and a couple other nameds (i think Lord Elgnub) nothing else sees invis.