Apiv, Agho, Aeth (Permanent Invisibility, Ghost, Ghost Visible): These abilities make units invisible without the tradeoffs of setting them invisible with triggers. Often this is a better solution than pausing units or adding locust. Units with permanent invisibility will lose their invisibility temporarily while they are attacking. So the ability can be used to create units like spirit wolfs at level three. Units that have both Permanent Invisibility and Ghost stay hidden, even while they are attacking. The Aeye (Sentry Ward) spell adds permanent invisibility to the summoned unit automatically by the way.