I used this list of the Dark Crystals to get them all. After getting half of these, the achievement should pop up.
These are the places where the dark Crystals are:
1 - The main audience chamber of the Tower. On a side balcony.
2 - The Minion Burrows area at the Tower. At a rocky dead-end near the blue
3 - The Minion Burrows area. A second crystal in the room, on a rope bridge
near the red hive.
4 - The private quarters of the Tower. On the pointed balcony.
5 - The Forge of the Tower, near the giant blower.
6 - Nordburg Sanctuary Cave. Near tree roots of "Beware the pointy hats of
doom!" human, near where you get your free red minions the first time. If you
warp here later, it is past the deep stream where you ditch all but your blue
7 - Nordburg Town. After using explosives to blow open Nordburg's front gate,
send a second mine cart into the village and detonate it. The cart will get
rid of some rubble with this dark crystal.
8 - Nordburg Town. Locate the ballista in the northeast (near where you
defeat Governor Borius) and go north past the locked gate. If you reload the
map after seizing Nordburg, locked gates with no linked turnstiles switches
are unlocked/opened. A path on the snowy mountains (the tutorial area) has
9 - Everlight Reef. Near the red keystone are some ruins your Overlord can't
get to because of deep water. Use blue minions to swim to it.
10 - Everlight Reef. After placing the yellow keystone on its pedastel (the
one you take from the elf ship). Your big ass Overlord needs to go down to a
sandbar infested with mermaids (the suffix of 'maid' is applied loosely here).
This is to blow up a ship blocking the path back to your ship (since you can't
use the elevator anymore). From the red and brown minion portals near the
yellow keystone, go down the first flight of stairs towards the sandbar. Stop
at the bottom of the stairs. Turn right and have the minions fetch the dark
11 - Everlight Jungle. Locate the pre-set greens. Locate a spur on the north
edge of the map. The road spur is next to deep water pond and it has a crystal
at its tip. This is after defeating the two fat Elves and near one of the
12 - Everlight Facility - North end of facility (ground floor) near where the
pandas were (but west) - uses a five minion floor plate. The Overlord might be
able to go this deep, but it's honestly faster if you are possessing a minion
13 - Everlight Facility - East wing of facility. Cave that leads outside to
some rubble waste area. You should get to it using the minions (faster and
14 - Everlight Facility - After using the possessor stone, there is a small
gate (opened with a five minion floor switch) you can open near where the
Green Hive as initially found. The gate leads to a space with this crystal.
You must be a minion to get this crystal, since the Overlard can fit his fat
ass up here.
15 - Everlight Temple - Having escaped the depths of the spider temple, there
is a secret room at the temple's exit with many treasure chests and a single
16 - Everlight Outskirts. Locate the spider webs near Netherworld Gate.
Requires a green minion with a spider mount to get it.
17 - Everlight Resort. Use blue minions to get to the island with the spell
catalyst. There is a small bridge only minions can cross. The crystal is at
the end of that small bridge.
18 - Empire Harbor. In a corner of the catapult fortress, ground level.
19 - Empire Heartland. Slay the Gargantuan after destroying all the Sentinels
on the main road for this crystal.
20 - Empire Sewer. Near the area's Netherworld Gate. Next to the raft you used
cross over the river, there is grating with the crystal's obvious glow shwoing
through. With blue minions, move the raft to a different dock, de-bus, and
fetch the crystal under the grating.
21 - Empire Sewer. At the crane room where you had to pick up and drop four
blue minions, there is a 20 minion floor switch. Get 20 blue minions to open
this side path and have your Overlord deal with the eradicators. The dark
crystal is at the end of the walkway.
22 - Wasteland. Southward pointing spur just north of the Wasteland's
Netherworld gate (gate 1). You can get this crystal once you have blue
minions; they suck up the magic goop surrounding it.
23 - Wasteland. Northwest quadrant near the zombie shire that's next to the
Wastelands Sanctuary entrance. In fact right before meeting Fay and Florian,
you may get this if you've been thorough.
24 - Wasteland. Northern zombie village. This is the zombie village (not a
halfling shire featuring the houses with round doors) near the magic pool with
the wasteland life upgrade. The crystal is behind one of the village's shacks.
Razing every building in the area like the 1st Cav in Aplocalypse Now will
uncover this crystal.
25 - Wasteland. At the big magic goop lake. Look for crystal in the lake's
northeast loop (it's rather hard to specify this one with zombies and fell
beasts trying to twist the nipples off your Overlord and all you have are le-
crap blue minions to keep the acid from burning off your face).
26 - Wasteland. During the escape from the Empire (with the Frank Miller 300
arrows coming at you on the cliff), you will scale an S-turn on a cliff. At
the top of the S-curve (pointing southwest) is a rise/side of the road only
minions can get to. The crystal on the side of the road before you slide down
one-way slope to the sanctuary. You can get this after the Tower Heart quest,
and the arrows will no longer harass you.
27 - Wasteland Sanctuary. On your way out after destroying all four shrines,
there is a red pit with fire arrow soldiers. Past them is a path leading back
out of the sanctuary. The crystal is on that path, in plain sight.
28 - Wasteland Sanctuary Town (one chance). As you go back towards Queen Fay
after destroying all four shrines, there is a point in the sanctuary town
where your minions push out two statues to shield the Overlord from fire
archers. Shortly after this, you will encounter a lone centurion (Roman leader
guy in red cape and gold armor) at an intersection of many stairs and
walkways. Take the stairs going up to this elven hippie lounge. The dark
crystal is hidden in the junk pile there. Note that if you complete the Tower
Heart quest (and take Dark Fay or Ghost Fay as the final mistress) you cannot
re-enter this area and dark crystal is lost to your game forever.
29 - Empire Hills. Past the second netherworld catapult, destroy the bombers
on the barbican (guard tower) by using wolf-riding brown minions to hop the
gap. The crystal is on the other side of the destroyed gap, in plain sight,
but hard to get to unless you have the right mount. If you need to change the
mount type to wolves, go back to the tower and make Kelda your First Mistress.
You can always change the mount type again after you get the crystal.
30 - Empire City. One of the city ramparts at the gate to Solarius' palace. If
you accidentally enter the Emperor's palace, you can turn around and leave to
get this crystal. Note that once you've beaten the game, you cannot go back
into the palace (or the arena, or the Sanctuary Town in the wastelands).
All credits go to Simalcrum at uk.faqs.ign.com. Here's a URL to the original list: http://uk.faqs.ign.com/articles/997/997038p1.html