improved fiendish servant optimization

[Soranar]

So I'll be playing a blackguard soon and while I'm still questioning how I'll be going about it (my DM waived the requirements if I play a paladin that fell so that's something at least)

game starts at level 12

Now I intend to take the improved fiendish servant feat to get an ape

my reasoning

-ape has 4 HD to start with and the highest STR
-with the fiendish template it becomes a magical beast and gains INT

now it should get 5 feats by the time it maxes out its HD (I doubt we'll be going epic)

the first feat I'm considering is a weapon proficiency (since it has two hands and claws are suboptimal)

the obvious choice would be a spiked chain (doubles its reach, now it gets 20 ft reach... not bad for a body guard)
armor proficiencies seem too costly, thinking of giving it bracers of armor

also I get to call it Booboo

that still leaves me 4 feats ... mage slayer?
and what skills can a magical beast take anyway?

[ExLibrisMortis]

You'll be doing something like paladin 1/blackguard 10/crusader 1 or warblade 1? There's not a lot of use in more paladin levels, or not trading them in, after all.

Fiendish servants get bonus HD, which may not grant feats, but let's assume your table rules they do. It starts at 6 HD, which gives it 6d10 hit dice (45 hp, plus its base Toughness feat), SR 11, DR 5/magic, resistance to cold and fire 5, and an iterative attack (with a Large weapon). That's pretty weak compared to your own 12 HD, and it's going to be even more behind later on (8 HD behind at level 20). With a spiked chain, you should be able to keep it out of danger, but remember to protect it with crusader maneuvers or buff spells.

Mage Slayer seems good, and you can take Martial Study and Martial Stance, or Shape Soulmeld. Try to pick a maneuver or soulmeld that complements your blackguard's abilities. For example, your ape can use Douse the Flames with a reach attack to facilitate movement by your blackguard.