I've been playing Terminus in Dark Host extensively for the past year and brought him to the WTC. Desecrator is definitely nice for spot removal, so if you like him, you should keep him, though I dropped him from the eventual list I settled on.
At least 1 arc node is not a luxury either. You will rarely use it, but it can still score zones and after you get a good feat turn, you might get an opportunity to assassinate with Hellfires. Post feat turn, it's also possible to get more Annihilations off, to keep you gorged on souls.
I like Stalkers with him, because they give some anti-healing tech and they are great for near-guaranteed 2+ kills on the feat turn, each.
Seether is kind of meh. At the beginning I had Barathrum with him, which is basically a better Seether, but he kept tripping over all the bodies, so I quickly dropped him.
Something else: min units is best units. Bane Warriors are slightly more expensive as minimum units compared to maximum, but you get extra opportunities for Officers, which means more recursion, which means more frustration for your opponent. In addition to this, min units have more autonomy with their targets and are less susceptible to debuffs, alleviating the need for Eilish.
I realize your collection might not allow this, so your mileage will vary.
Machine Wraiths are golden, never leave home without at least 2.
Darragh is great, but not worth dropping the six bodies of a min unit of Knights, sad as it is.
Win condition is your Banes killing opposing hard targets that could threaten Terminus and then Terminus running the board. Alternatively, just use the Banes to screen so Terminus can kill targets himself, ideally do this on feat turn so you're invulnerable. Many good feat turns start with Terminus flying over his screen, breathing 2-3 poor chumps to death and then killing a heavy. Rest of the army kills 4+ more chumps and you're a god at ARM 24+
The list I still use, your mileage may of course vary:
conflictchamber.com/#c4201b_-0B0W191h1h1hi22k2ki4i4hZh_hZh_hZh_bBCryx Army - 75 / 75 points
[Theme] Dark Host
[Terminus 1] Lich Lord Terminus [+27]
- Deathripper [6]
- Stalker [8]
- Stalker [8]
- Stalker [8]
Bane Lord Tartarus [0(6)]
Machine Wraith [2]
Machine Wraith [2]
Bane Knights (min) [9]
Bane Knights (min) [9]
Bane Warriors (min) [10]
- Bane Warrior Officer & Standard [0(5)]
Bane Warriors (min) [10]
- Bane Warrior Officer & Standard [0(5)]
Bane Warriors (min) [10]
- Bane Warrior Officer & Standard [5]
Wraith Engine [15]
To actually critique your list:
If the Bane Knight Officer is the same as the final CID version, I would not take him with Terminus, as the Defensive Strike + Take Down is too much of a headache to factor into your own feat turn.
Drop the Seether for 2 arc nodes, or try to fit in a Stalker.
I would try to "upgrade" 1 unit of Knights to Warriors. The latter have lower ARM, yes and the Knights get Tough from Terminus, but Rise and the mini-feat are still very valuable.
If you like Darragh, or your collection does not allow you to drop him, you should keep him.