I’m from PoE land and they are very specific about how on-hit effects work. But I don’t seem to be able to find any info on it for Grim Dawn. Will bloody pox, a DoT spell interact with Shepherd’s call? Maybe someone can give me a rundown on “On-hit” effects?
Firstly welcome to the forums and the beautiful world of Grim Dawn 
Yes it will work but not 100% though. Meaning that the effectiveness depends on the skill you will use and/or its cooldown to activate it might change the chances to activate.
Lets Say the constellation skill Murmur mistress of Rumors says that :
Rumor (%) Chance On Attack
Now If we say I use the Nightblade’s Skill Amarasta’s Blade Burst which has 1.8 cd the % value is at 27% chance on skill’s use.
If i use lets say Beronath’s Fury skill which has no cd then the chances drop to 15%.
Check out your skills and play around to connect with your playstyle of your build
About the triggers it works as bellow
There examples like these on Constellations :
(%)Chance on Attack
It has the % chance to activate with your assigned skill
(%)Chance When Hit
It has a chance to activate when YOU get hit by anything
(%)Chance On Critical
Its mostly 100% when you land a critical hit with the skill assigned to it
(%)Chance On Enemy Death
It has a chance to activate when any of your dead targets dies
(%)Chance On Block
It has a chance to activate when YOU block
(%)Chance when Hit by Melee Attacks
Same story with the “When Get hit” instead it has to be melee attack from enemies
Thank you for that! very helpful. I tested it and Shepherd’s Call procs constantly with Bloody Pox. Quite enjoying it so far. Also, does this game have “Double Dipping” Mechanics? Such as buffing your %Physical damage also buffs your bleed damage done? ty again!
No, +% Physical damage or anything that specifies Physical specifically are specific to Physical damage. (e.g. -x% Physical resistance that is present on the skill Curse of Frailty for the Occultist).
Bleeding damage will have it’s own % damage ,resist reduction, etc.
Edit: There is a stat for +% to all Damage for example, Squad Tactics for the Soldier gives a source of this which as you would imagine effects all types of damage dealt by your character such as Fire, Acid, Pierce etc.
The one thing to be aware of is the difference between + elemental and +% elemental damage.
With + elemental the damage is split between the 3 main types so 9 + elemental is +3 cold, +3 fire and +3 lightning.
With +% elemental each of the 3 elements receive the full bonus so 9 +% elemental is +9% cold, +9% fire and +9% lightning.
I see, then how do pet’s scale? off my base stats or through only Pet specific modifiers found in celestial buffs or skill tree?
Pet specific modifiers only. These will usually be listed underneath a header that says “Bonus to all Pets”. You can see this in constellation nodes and celestial powers such as Shepherd’s Crook, Jackal, Panther, Manticore, Bysmiel’s Bonds and many more as examples.
You can also buff pets with auras. Buffs with a radius to them such as Blood of Dreeg or Flame Touched will also effect your pet evident by them receiving a graphical effect or if they have a character window, the buff icons will appear beneath their health and energy bars.