Infusion

Infused armor - what does that mean? Just got my armor infused and knew I could stand up to those crystal baddies now, but what are the actual benefits? Don't know if I missed something in the cinematic, cause my team was in a real hurry... of course. Also if I change armor do i have to get that infused again??

Reduces the effect of the mursaat spectral agony.

Each time you add/replace a piece of armor it will not be infused and so will need to be if you will be facing mursaat.

OK - thanks for that. I wasn't sure if it affected other types of damage or not.

You don't need to have all pieces of your armor infused. The more equipped armor pieces that aren't infused, the more damage you take.

I think it is -9 spectral agony dmg for all pieces infused.
-16dmg for 4 pieces infused.

Yes, Infusion only applies to the current set of armor you are wearing when you select "Accept" when talking to the Seer after returning the Spectral Essence.
Just as a side note, not sure if you knew or not, it is possible to get infused in several other places other than Iron Mines of Moladune. To my knowledge there are Mineral Springs(northeast of the cave where the IDS's drop), Ring of Fire(near the first few groups of Ettins you encounter), and Abaddon's Mouth(near where the bonus takes you). In my opinion, Iron Mines is still the easiest and quickest way, but just thought I'd let ya know

There is also a seer in Hell's Precipice, though I've never used it.

There's actually five of them in the game. One more in Mineral Springs, and one in each RoF mission.

The seer in Hell's Precipice doesn't infuse you, he/she/it just gives you the bonus.