I'm the sacred hydra wielder in this case, and those ghoul guardians are a nightmare. I thought I had it made against his E9 blessed templars, which were hammered by the hydras, but jazzepi pulled a blood marshal hero early, massed ghoul guardians and set to against my hydras. I didn't expect him to be able to field such large numbers of poison resistant troops so early. It gave him a strong edge for several turns, now its kind of a weird situation where that army kicks ***, but if I send my hydras anywhere else they have an easy job against Ulm's armies, whose high protection is largely irrelevant
I don't think an available recruitable undead commander is really necessary. As has been mentioned there are summons available, or a death pretender can be taken. Sacred hydras are a very particular threat with a lot of inherent risk, as poison resistance very very heavily neutralises the strategy, so they will typically only be very effective for early expansion (if not for that damn blood marshal!
). A specific unit for such a specific threat seems unwarranted to me. Anyways, a good undead leader is clearly available through luck, so the player can mitigate the risk through scale as well as pretender selection.