I posted this in the Empire Strikes First thread but it sounds like I can add these pics here too. Onizuka posted a video of the one at Horizon in that thread too which is fun.

This is Roysia the Tranquil NPC

http://imageshack.us/f/14/ffxivgame20110723193041.png/

I think the gist of the text is the same as Alfgar.

Here's the Militia Outrider:

http://imageshack.us/f/94/ffxivgame20110725150502.png/

This is the NPC you have to speak to to take part and receive the reward from. He appears at 37:48 in the South Shroud - I've seen him there twice anyway.

There's 2 achievements - one for talking to Roysia or Alfgar about the attack (The Empire Strikes First) and then one for taking part (A Taste of Victory).

Roysia appeared for around 30 minutes and shouts periodically too.

Here's a picture of the imperials

http://img38.imageshack.us/img38/753...0725150437.png

The Militia Outrider was on the other side of the tree in the picture.

You don't gain SP though, but I received 7 deaspected crystals for taking part. On the two occasions, it was around 7.30pm on 23rd July (GMT+1) and 3.30pm on 25th July. Been too busy to play since alas. Hope that helps!