I’ve had very little to do with the skirmish system since it’s advent in LOTRO. On paper it is a system of scaleable dungeons which allow players to jump into an instance from anywhere but also to set the group size and level of the dungeon within certain limits. This really empowers the player to fine-tune difficulty for small group or solo instanced content.
Some of the Moria instances are (or have been converted into) skirmish instances. It’s pretty unusual for core story content like this to be offered in such a variable format.
Skirmishes have a certain common formula to them. There are a small number of locations to defend. The enemy forces appropriate to the particular quest story will appear in waves to attack one of those locations at random. The player(s) run back and forth fighting off the attackers to ‘win’ the skirmish. There are usually a few extra elements to a given skirmish, e.g. the trilogy of skirmishes at the end of book five of the Moria storyline have banners lying around in the various locations that offer bonuses to the players or penalties to the attackers.
There are also some bonus bosses that spawn seemingly at random during skirmishes. These are not connected directly to the attacking waves. It is however easy enough to accidentally pull one during such a wave. This troll for instance appeared in the final room of the middle instance – right before the end boss spawned with two followers. So I suddenly had two trolls and two orcs beating on me and despite a heroic attempt at kiting and dragging the dwarves from the main room behind into the fight I died. Compared to the difficulty of the general skirmishes on solo mode this was a bit of a shock. Best to be at least a bit cautious until you know the instance properly!
Despite the heavy focus on fighting big bad orcs (or Angmar or whatever), there is the odd curve-ball type encounter. This treasure stealing dwarf was a nice example; LOTRO is a heavily lore-constrained world where monster variety is concerned so thumbs up to Turbine for trying to vary things within those constraints.
After completing the trilogy I suddenly was sent to the First Hall to go find some Elven allies. Without any more ceremony my champion was standing out in the fresh air looking at a starlit sky for the first time in many, many gaming sessions. It was a great moment!
The skirmish system is pretty powerful at least in the control it allows players over the relative difficulty of an instance for their character or group. It does also mean more variety for alt-play, given the random elements if you just run these once per character as you level up then you’re unlikely to see all the slight variations.