During a livestream I watched, one of the developers confirmed that there are secret achievements in The Darkness II. Xbox360Achievements also lists several secret achievements, so I assume this to be true, at least for the Xbox 360 version.
What are those, and how do I unlock them?
Xbox360Achievements actually lists their descriptions now, and I've unlocked them on Steam as well, so here's the deal.
The only true secret achievement is:
Decisions, Decisions - Refuse to cooperate with Victor in the Interrogation
At one point, you're forced to sacrifice either Frank or Eddie. If you don't decide by not pressing anything, one of them will be killed anyway, and the achievement will unlock. Not sure if it's random, but he killed Eddie both times I refused to pick one.
All of the other secret achievements are pure story achievements. If you play the campaign, you will get them, and going into too much detail would just spoil the story (which is probably why they're hidden in the first place). These are:
The last one is also a story achievement, but for the Vendetta campaign (more specifically, for finishing it).
Should Have Called 555-2368… - Survive the battle against the Hell Beast
The phone number is a reference to Ghostbusters, by the way. And, just so you know, there's a Hell Beast at the end of Vendetta mode. I bet I just totally ruined the complex and involved story for you.
Since in the Steam version the developer can't hide achievements, it's clear what they are, see here: http://steamcommunity.com/stats/67370/achievements
The ones without description are the secret achievements.
I will list them here and update the answer when I know how to unlock them:
