Invoker’s Summon Damage Bonus Effect
Invoker is one of the two Summoner focused mod traits that affect the mods that create creatures that fight on your behalf. This trait basically takes the place of Evocation, since that trait doesn’t affect summon mods with it’s damage buff. On the plus side, you get 5% more bonus to damage, since Invoker caps out at +20% versus Evocations +15% buff.
Update ‘Subject 2923 Pre-Release Patch’: Looks like summons now get the bonus from regular mod damage now too, making Invoker considerably more powerful, but should probably still be prioritized after Evocation. This mod also no longer requires the Swamps of Corsus DLC.
How to Find the Invoker Trait
To unlock Invoker, you need to collectively score 100 kills through the use of your summons. This can actually take some effort as most summons don’t end up laying killing blows, which are needed for this to count, due to their slow attack speeds relative to most player weapons.
I would recommend using two weapons both equipped with summoning skills (I used Seed Caller and Beckon) and run around an area with lots of weaker minions to kill, like the outdoor areas in Corsus. Try to fire as little as possible, or just weaken most enemies so your summons can score the kills they need. It took me about 10 minutes or so of doing this in Corsus to unlock Invoker.