I’m on a roll now with tables of boss frequency. So here’s the numbers from wowhead for frequency of bosses appearing in Violet Hold heroic.
The last boss (Cyanigosa) will always be there. But the two other minibosses are ‘randomly’ selected from among the other possibilities.
| Nr Times Looted | Boss Name |
|---|---|
| 7419 | Lavanthor |
| 8908 | Erekem |
| 9037 | Zuramat the Obliterator |
| 9463 | Xevozz |
| 9786 | Moragg |
| 10054 | Ichoron |
| 22031 | Cyanigosa |
The numbers don’t quite add up. Two minibosses must have been killed for each instance of Cyanigosa (because if they weren’t, groups would not have got to her), but if you add up the minibosses looted it comes to more than twice the number.
This just means that not every group who started the instance was able to finish it.
But still, Lavanthor does seem to show up a lot less than the others. And it isn’t because he’s a particularly hard boss and so lots of groups would fail to loot him for that reason. He’s not.
Or maybe I’m just narked because he has a tanking trinket that I want, but I have only ever seen him once in 11 runs.
Welcome to Spinksville! 
It also is a reflection of the random number generator that Blizzard uses. Truth be told, there is no such thing as a truly random number generator just increasingly sophisticated ones, so that certain bosses would show up more than others is indicative of the lack of randomness in the system.
This kind thing is very important for encryption. Probably less so for boss rotation/loot.
WoWhead reported numbers can be very quirky – if all five members of a group submit their logs, the database will log the same kill five times. You wouldn’t expect that to skew the data in the long term, but it probably drives the required sample sizes up a few notches.
Personally, I had 10 Cyanigosa kills, all the relevant mage loot from the instance, and various failed runs (with one or two sub-bosses downed) before a save finally graced me with Erekem for the achievement for killing all six subbosses. I feel bad for anyone who is bent on trying the achievements for killing specific sub-bosses, it must be irritating to have to wait for days to get a boss and then have the group kill the boss without fulfilling the achievement requirements.
I hadn’t really thought about the achievements but that’s true, and also if you wanted a drop from a particular miniboss but got unlucky, you could have to repeat the instance way too many times (in my opinion 🙂 ).
On the other hand, I do think the unpredictability of ‘which ones will we get’ probably does add to the fun here. If only they could have somehow decoupled that from the loot/achievements a bit.
Most pugs probably fall apart on the Shadow spam boss in Heroic mode (forget his name).
I’ve been running it almost every day for 3 weeks. I’ve only actually seen Lavathor himself twice.
I feel like the talisman will never drop hehe.