The Harbinger: High Power, Low Cost [Harrow]

FEATURES:
No Forma Cost
No Primed or Umbral Mods
No Helminth Required
No Mandatory Arcanes
No Focus Schools Required

HOW TO PLAY:
[Harrow] spawns with max energy (375 in this case). We immediately activate his 2 and 3, run to enemies, hit them with 1 to gen shields, press 4 to activate 11s invuln window, kill one to proc his 3 if you need energy, recast 2 to spend shields to get a huge amount of fire rate/reload speed, then recast 1 on enemies to refill overshields while still in invuln window. After that, just refresh 2-3-4 as needed, and use condemn at will to recharge shields and/or CC enemies.

SYNERGY EXPLANATION:
Power strength gives us more fire rate, crit chance, etc. Duration gives us more time on buffs - the more duration, the easier to play. Range is only here for 1 (Condemn), so we don't need a ton of it. I like Stretch in this build because it puts us at 29m for Condemn, but this could be swapped off if there's something else you want to run for more mobility (rush) in speedruns, holster procs for snipers (vigorous swap), or Rolling Guard for an extra "oh sh*t!" button. Augur Message can be swapped for any of these as well, if you need the range but want some of the utility items I just listed.
Thurible gives us more energy than we know what to do with, so efficiency isn't an issue. Penance gives us over 9% lifesteal from ALL sources, so we can outheal toxin procs without needing higher than base HP. Covenant gives us a long invuln window so we can safely recast Penance and [Regen] shields with Condemn before being vulnerable to damage (Rolling Guard also works in a pinch if you decide to swap that in for one of the flex slots).

UPGRADE OPTIONS:
This build is an incredibly strong starting place with very little investment, but the first place I'd look into upgrading would be swapping Flow for Primed Flow and Continuity for Primed Continuity. These both just make the playstyle/rotation much easier to maintain. As for forma, I would say go for an aura forma or Y polarity in the aura slot first, so that we can use the Combat Discipline/Arcane Avenger combo, which synergizes very well with all the crit focus [Harrow] has in his kit.