I was thinking of picking one of the more heavily armored Trolls like the Troll Chieftain or the Isengard variant.
I'd give them a dark skintone and golen armor. Would they be a useful addition to an Easterling Force mostly composed out of infantry, archers and one cavalry wing? And if so... should I deploy them in 3's?
Finally, what Trolls rulewise would be most effective?

Which Nazgul would be best suited to lead 5 Morgul Knights?

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A shadow rises in the East...
The Easterlings march to war!!!

I like to be aggressive with knights, so I would choose a ringraith that makes them deadlier. My top two choices are the Knight of Umbar and the Betrayer. The Knight of Umbar's ability can give the formation a higher fight and it also makes him better in duels. The Betrayer would also work well as he would make the knights much deadly. This being said, all of the ringwraiths would work well in their own way. Most of the other wraiths have more defense abilities. However, you could argue that the knights are deadly enough on their own and some defense would help keep them alive to kill more stuff.

You don't need trolls, but if you want to field them they will work fine. Taking several of them works best. Isengard trolls are slightly more durable, but they have lower fight. I am not a fan of the troll captains, as I prefer to take more trolls. Given that you like armor and that it fits your theme, I would go for the Isengard trolls rather than a regular troll chieftain.

Yeah but I might not use the Troll Chieftain as a 'chieftain'.

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A shadow rises in the East...
The Easterlings march to war!!!

The Dark Marshal is a fitting and sensible choice for the Morgul Knights (may be best to wait until the plastics are out though ), Khamul would fit the rest of the army and help to keep them alive, but the Knight of Umbar and Betrayer would also both be effective (although the Betrayer is best for archers IMO).

As for Trolls, Mordor Trolls (whether 'normal', drummer or chieftain) seem to be best, IMO. If you do use one as a chieftain, then you can have all Trolls in 6" make Heroic Charges, very useful

Yeah for the Morguls i'll think I'll wait. I already have 7 Kathaphrakts. For now I'll put Khamul in command of them.

I do hope the Dol Amoth / Swan Knights minis are all turned into plastic kits!

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A shadow rises in the East...
The Easterlings march to war!!!

They will be, in not-so-far future...

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"There are few left in Middle Earth like Aragorn, son of Arathorn." - Gandalf, Many Meetings

When they come out. I WILL start a Dol Amoth WoTR Army.
It shall be... GLORIOUS!

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A shadow rises in the East...
The Easterlings march to war!!!

What about a troll chieftan, and then spending the rest of ur allied points on cave trolls? you would be using 25 less points per troll, enough for a company of haradrim or somthing

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One ring to rule them all, one ring to find them, one ring to bring them all and in the darkness bind them

Glorious... but small. They're 50pts a company...

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"There are few left in Middle Earth like Aragorn, son of Arathorn." - Gandalf, Many Meetings

Look on the bright side! Little to paint!

_________________
A shadow rises in the East...
The Easterlings march to war!!!

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"There are few left in Middle Earth like Aragorn, son of Arathorn." - Gandalf, Many Meetings

Not big on Haradrim. ^__^

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A shadow rises in the East...
The Easterlings march to war!!!

The biggest Problem with Morgal Knights is the limitation of 4 companies in a single formation. Putting a Ringwraith in such a small formation can be risky. I think a captain is better because you have 2 might points to call heroic fight which is extremely powerful with calvary. Ringwraiths have only one might which typically needs to be saved for epic strike.