Big Blind Bill makes some excellent suggestions I would definitely take into consideration what he is saying.
I was trying to suggest that PFG librarian on bike w/ command squad because I think it is cool, competitive, good against tau, and fits well with the
DA fluff. Plus, if you use their teleport homers to usher in the Deathwing that makes a lot of sense as well. Plus, I think black knights are one of the "coolest" units in the army but my taste might be different than yours.
An alternative if you want more terminators is to take a Librarian (or even just Belial) and a command squad that looks like this:
Command Squad
- Champion w/ Halberd of Caliban
-
TH/
SS
-
TH/
SS
-
TH/
SS Banner of Fortitude (for
FNP bubble)
- Power fist + Assault Cannon or Heavy Flamer (depending on preference)
If you put these guys with Belial, they can deep strike with no scatter, right where you need them to give
FNP to everyone you need. Deep strike the rest of your terminators first, see where they scatter, and then pinpoint the banner so that it gives
FNP to all your dudes on turn one. Twin link that assault cannon and all the other shooty terminators that dropped in and you should have around 15 twin linked,
FNP terminators in his face on turn one. Make sure to have at least 1
TH/
SS in the front of each squad to give you that 3++ against any apt for as long as possible too

It might not win but it's damn cool haha.
You can also add in a librarian if you want in order to hopefully give some divination powers, effectively letting you have re-roll terminators every turn of the game. You could also give him a PFG if you want to give a shooty squad a 4++ and have the librarian bop around from squad to squad. However, be aware that the PFG confers it to ALL MODELS within 3" including enemies. So best to keep him at the back of combats to only boost your models, or have him leave squads right before they charge in so as not to give tau a 4++ against your power fists haha.
- VardenV2