Reaper?

[Pocus]

I'm intrigued by a barbarian Reaper, which can be seen during move but then reverts to invisible mode, any guess?

 

[MrUnderhill]

It was probably a Wraith summoned by Barbatos the Lich. The reason it disappeared is that, like most summons, they typically only last one turn

 

[MagisterCultuum]

IN addition to being summoned by the Death III Summoning spell, Ars Moriendi (the Fourth Horseman) also creates one whenever he kills a unit. The Wraith do not have limited duration.