I am not great at CoH m,echanics. But here goes
From the HC Wiki (and it may be 15 years out of date, they never change this stuff...
"While active, you channel Negative Energy onto a targeted foe. Darkest Night decreases the damage potential and chance to hit of the target, and all foes nearby, as long as you keep the power active. You must be level 41 (level 38 for Brutes/Tankers) and have Gloom or Soul Tentacles before selecting this power."
From MiDs: 25 ft radius, -15% tohit, -30% damage
The -damage ans -to hit will apply to all enemies in a 25ft radius of your target. It is a toggle.
This power is GREAT for 1 thing. Irritating the shit out of everyone around your toggle target. On a Tank this is pure gold. Especially a Dark Tank that has a stealth aura. You are NOTICED.
This power is good (who cares much?) at debuffing the damage (I am on a Dark Tank, I give a shit about the dam,age you do) and the to hit. Seriously. My Dark Tank is a Obelisk of I do not care. The power is taken for it's incredible agro management abilities. Modern teams want to debuff Regen and Resistance, sometimes defense, but debuff Regen/Resistance is the gold mine. Having a power that debuffs damage and to hit is nice. nice. Not a gold mine. Agro Management on the other hand. F Yea!
These are my opinions. You can find an army that will tell you I build poorly and play worse. Some days they are right.