The reason he sees thru invis/undead, and not thru invis, is that he's not undead. There are 2 of these, one on the docks, one in the tunnel, and they will ruin your day if you're sneaking around with invis/undead up. They don't path back to their spawnpoint...they corpsecamp/zonecamp/etc... I've trained the one on the dock to a corner, died there to leave him there, then had rogue get corpse...and leave treasure sorter way out of the way somewhere til next patch..makes the naked CR's a lot easier
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Toucan Tanksem of Vallon Zek, Sebilis, Venril Sathir, Antonius Bayle, and now Toukan of Povar
My experience shows that these do not see invis. At least I've yet to encounter one that does. It's possible you were roaming around near the soaked/water logged/ or other undead type trolls that occasionally see invis and this guy happened to be in the area causing him to frenzy aggro.