I had a lot of fun hunting down the Fallen that have been placed around the different areas, sometimes replacing previous spawns. A few of them are on the usual patrol routes, so you can't really miss them (though one of them has a nasty tendency of finding you before you find him!); others are off the beaten path and require a bit of exploring to find.
Some of you have no doubt found guides online that reveal each of these (sometimes) elusive targets, probably in order to fill out your Grimoire. But if you haven't yet done so, I urge you to go out and find them yourself! The fights aren't really difficult at all, but they can break up the monotony of patrol. Additionally, killing the yellow targets grants you a Grimoire card and - [i]sometimes [/i]- a Token for use with Variks later on.
I didn't realize they were there until I was ambushed on my way to the Divide in the Cosmodrome. After figuring out what that was all about, I started hunting down the rest of them. I managed to find all of them except the High Servitor, which turned out to be in a place I thought had been checked by default! For shame...
Oh well! In any case, the Queen wants her due. Go drag it kicking and screaming out of some Fallen!
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There's a fallen outpost near where you wake the hive on the moon. Accidentally saw a 28 captain on the way back from a patrol quest and found it, neat addition I think :)