Yeah, getting the death mages is easy, it's getting enough death gems to spam them that's hard...

You need 10 death gems per casting... and while 2 Baghests for the price of 1 death gem is indeed a steal, it's still hard for Man to pull off.
The example I pointed to is just so that you can start casting 20 Baghests every turn in the first year (all you need is a little luck with site searching and level 2 conjuration.)
Edited to add stats (for those who haven't had the chance to check):
Summon Black Dogs = Conjuration 2, 2 death
Summon CU Sith = Conjuration 3, 2 nature
Summon Baghests = Conjuration 4, 2 death
If everything worked normally:
Summon Black Dogs should summon 20 black dogs for 10 death gems.
(Black Dogs: 14HP, size 2, 4 Prot, 13 Mor/MR, 2 Enc, 12 str, 12 atk, 10 def, 5 prec, 3/24 Mv, +20 Stealthy, Animal, 100 Darkvision, Bite attack)
Summon Cu Sith should summon 7 Cu Sith for 10 nature gems.
(Cu Sith: 26HP, size 3, 7 Prot, 14 Mor/MR, 2 Enc, 15 str, 13 atk, 12 def, 5 prec, 3/28 Mv, Sacred, +0 Stealthy, Forest Survival, Animal, Bite attack)
Summon Baghests should summon 9 Baghest for 13 death gems.
(Baghests: 28HP, size 3, 6 Prot, 14 Mor, 15 MR, 2 Enc, 16 str, 13 atk, 10 def, 5 pref, 3/26 Mv, Sacred, +20 Stealthy, Animal, 100 Darkvision, Venomous Bite attack)
Both the Baghests and the Black Dogs are suppose to be stronger in darkness according to the description, but I haven't tested if that's something other than the 100 Darkvision.
The big mix up here (as I posted at the top) is that instead of 13 death gems for 9 baghests, we're getting 10 death gems for 20 baghests, which is around 3 times more effective than it should be, and 2 level of conjuration earlier... Combine that with Man's bards/monks and you've got one nasty stealthy strike force (just be glad they don't come with better leadership).