I would take out nightshroud, the scientist, shadow reflection, and deathfire crescendo.
Nightshroud is just bad and doesn’t proc enough to be worth it, especially as death knell is random.
The scientist is fairly terrible, just run blue conjourer instead.
Shadow reflection is an aggro card, this is a control deck so it doesn’t fit here.
Deathfire crescendo is a swarm card that requires stuff to die, owlbeast gives stuff more health so they don’t die, this kind of anti-synergy is to be avoided.
Now that the bad cards have been taken out I would add:
Sphere of darkness, any 1 cost card that draw a card is excellent, this also gives a ping which is super useful for trading, a 3 of in any deck even without the need for cheap spells.
Deamonic lure, one of the best removal spells in the game, also combos with sphere and your general for stuff like circulus, kelino and other backline annoyances.
Void pulse, another cheap spell for procing owlbeast and illusionist, also good as a finisher and for emergency healing.
Spectral revenant, this is a contender for best minion in the game, along with makantor, lavaslasher, falcius, azurite etc
Blue conjouer has already been mentioned, it is kinda broken in these kind of decks.
These are core cards that you need in this kind of deck.
Other things to consider are grasp of agony (can be a great board clear with deamonic lure), lightbender (as an alternate for ephemeral shroud, it has arcanyst synergy and is aoe, but is more expensive. It’s your choice but I think it is a better fit), alcuin loremaster (this is more situational, but can be super strong is you steal trinity oath or something).
Some other things to consider is to change the number of certain cards, specificly death knell as that only really needs to come once and if you are going late game then you can go through most of your deck. You are also kind of lacking in terms of 2 drops, missing turn one is really bad, adding some more is a good plan (stuff to consider are healing mystic, primus fist, and gloomchaser).
As a disclaimer I despise abyssian and don’t play them, so most of what I just said is based on general game knowledge and what I have seen and may not be fully acurate.