Slaaneshi warlord says "hi!" This means cheap units can't hurt him, he's got AE for tokens, and is tied for the most HP amongst any warlord in the game, for dealing with other warlords and big units. Not just a Ku'gath card.

The text for Reaction: is wrong.  The card reads "Daemon UNITS" not "Daemon ARMY UNITS"

That's a huge difference considering it's now extremely relevant with the reveal of Vha'shaelhur.  Now both him and Ku'gath can be protected by this event.  Combine with it being a 2 shield card makes it great in my opinion.  Both Ku'gath and Vha'shaelhur will want to do a lot of their own fighting so they need defensive cards like this one.

So Ku'gath's Nurglings would be immune to their own move forced reaction, yes?

If you DS before they move to that planet.

Other way around.

Remember that events only interact with the units that are present when the event is played (or DS'd in this case). So the only units that gain the "cannot be damaged" limitation are the ones at the planet when you DS the event. Any Daemon that moves to the planet after you DS the event will not be affected, and so can still be damaged by units with printed cost 2 or lower. Any Daemon units that were there, but then move away from the planet would carry the damage limitation with them. 

Think about which Eldar units get the +1/+1 from Empower. This event works the same way (just with a specified planet instead of a chosen target planet).

So, if you DS the event at Planet A, then move a Nurgling to Planet A, it is still going to damage itself. But if you DS the event at Planet A, then move a Nurgling who was there when you did to Planet B, it would not damage itself.

Duh, forgot about the timing of DS.

...Any Daemon that moves to the planet after you DS the event will not be affected, and so can still be damaged by units with printed cost 2 or lower. Any Daemon units that were there, but then move away from the planet would carry the damage limitation with them... 

The card seems to read that any daemon at the planet, regardless of whether it was present prior to the deep striking (deploy phase deploy) or deployed after the deep striking (i.e., ambush) would benefit from the damage limitation. 

The card seems to read that any daemon at the planet, regardless of whether it was present prior to the deep striking (deploy phase deploy) or deployed after the deep striking (i.e., ambush) would benefit from the damage limitation. 

That would be true if the event resolved on the planet itself (similar to Rok Bombardment). But it doesn't. It resolves on the units at the planet rather than the planet (like Empower).

Essentially, the effect places a lasting effect on "...units (at this planet)...", not "...(units at this) planet...". Always look at the object of the sentence to see what is being affected by the ability. "At this planet" modifies "units" as the object of the sentence. The use of the word "each" in Empower is clarifying that that the units are affected, not the planet (which is an actual target of the effect). This event doesn't target the planet, so the word "each" isn't necessary, but the construction of the two abilities is the same -- with the units being the object that the event affects, not the planet.

Because the event resolves on and affects the units - not the planet/location - its effects are limited to the units that are present when it is resolved.

That would be true if the event resolved on the planet itself (similar to Rok Bombardment).

The problem is that, from the wording, it looks like it does resolve on the planet itself.  A better way to word this would have been, "After you Deep Strike this event, all Daemon units you control at this planet gain "Cannot be damaged by units with printed cost 2 or lower" until the end of the phase.

Other way around.

 

Remember that events only interact with the units that are present when the event is played (or DS'd in this case). So the only units that gain the "cannot be damaged" limitation are the ones at the planet when you DS the event. Any Daemon that moves to the planet after you DS the event will not be affected, and so can still be damaged by units with printed cost 2 or lower. Any Daemon units that were there, but then move away from the planet would carry the damage limitation with them. 

 

Think about which Eldar units get the +1/+1 from Empower. This event works the same way (just with a specified planet instead of a chosen target planet).

 

So, if you DS the event at Planet A, then move a Nurgling to Planet A, it is still going to damage itself. But if you DS the event at Planet A, then move a Nurgling who was there when you did to Planet B, it would not damage itself.

That´s correct but the Nurglings ain´t the one that does the dmg, it´s a card effect.... just like AE is a card effect.

The problem is that, from the wording, it looks like it does resolve on the planet itself.  A better way to word this would have been, "After you Deep Strike this event, all Daemon units you control at this planet gain "Cannot be damaged by units with printed cost 2 or lower" until the end of the phase.

Perhaps, but despite the sub-optimal wording, this effect does resolve on the units, not the planet. But send it in to FFG if you want to be sure.

That´s correct but the Nurglings ain´t the one that does the dmg, it´s a card effect.... just like AE is a card effect.

That would only matter if Prince's Might said the unit couldn't be "...damaged by an attack from units with printed cost 2 or lower...". Damage from a card effect - including AoE - is still said to be coming from the card. Otherwise, "immune to events" wouldn't do anything because its the card effect, not the event card, hitting the immune card.

The damage from AoE may be "effect damage" instead of "attack damage," but it is still considered damage done by a unit.