well out of all drk ability's, there are a lot of posibility's for new 2 hour that could work.

Zerg style 2 hours

Grim reaper = ( or just Reaper ) a sustained death effect on evry hit, chance of procing death increase evry hit, when death is not procced a dark magic element is added to strike starting at low dmg and increasing by some apropriat amount evry hit but TP is drained to 0 under duration of 2 hour effect
should be locked to 2 hander only so no club spams.
last 1-2 min

Blade storm = maxed haste ( 80%) Quad Attack 80% for 30sec-1min duration but tp is drained to 0 on evry hit to boost dmg, perfect for Zerg and opens for gearing towards str/attack/critt/TP under zerg, a perfect tool for high delay/high dmg 2h wepon.

Level ??? Death = instant 2 hour, attempts to hit enemy with death, drains all mp/tp and if it fails does massive dark magic dmg ( can be boosted with MAB/int gear and ability's too boost magic ), Amount mp/tp directly increase chance of death/amount dmg if fail.

Death incarnate = turns u into a corse with full zombie status ( meaning dmg heals u and cure dmg u ), gives u access to special dark magics and boosted mp pool for duration but melee dmg is weakend as attack is converted into MAB.
Duration 5-15 min, TP is converted to MP evry 3 tick
Dark Magic under effect availible are : Ruin = looks like gravity/demi on animation, but does tier 4 elemental dark magic dmg.
Ruinga = ga version of ruin does tier 3 elemental dark magic dmg.
Death = death spell, may or may not proc death but does dark magic dmg if fails.
Doom = hit target with doom countdown from 10, may be resisited and do dark magic dmg instead.
Black hole = a powerfull demi spell will do 1/16 1/8 or 1/4 dmg to enemy existing hp, dark elementh based.
Dark Siphon = Status on target that adds endrain3 to all attacks done on target.
Singularity = Multi hit dark magic spell doing 2-8 hits on cast, each hit increase dmg of all dark magic on target, leaving a lowered dark resist on target mob, base dmg of first hit is 300ish
Absorb-MP = takes enemy mp and adds to own, if full mp it gives mp-max effect for extra mp aspired, fast recast.
Absorbra-random= Takes random stats from enemy target and gives to all party members in 15" range of mob
Drain 3 = takes large amounts of HP of target and adds max-hp to u if u drain more then u can hold.
Ragnarok = Uses all hp/mp ends 2 hour effect by killing u for no exp lost and short weakness, Does massive 8 hit dark magic attack based on mp/hp u have + magic stats, aim is to weaken mob to dark magic and bolster mp/hp before using this final dark magic attack, adds a imunity effect to target to ragnarok if it survives for duration of fight, but can be group cast like meteor for 1 mega hit.

Level x100 strike = does x100 amount dmg of curent level so if 99 it does 9999 dmg to target instant with main wepon.

Death Risen = summons Corse minion to ur aid, it will cast dark magic on all targets u engage.
Has high level of fast cast and loads of mab/int + access to all dark magics, does not melee at all.
does Leech-mp on master ( u ) evry now and then to keep mp up, prioritize dmg spells on targets as long as mp is high,
once mp hits 0 it does a kamikaze magic attack attempting to doom target prey, if fail it does dark dmg instead.
Duration 10-15 min as long as mp is refilled from drk, magic cast from corse should be on par with blm only it is dark based.

as for non zerg or utility 2 hour im sure there are some that can be used

Knight Terror = grants a boost to dark knight maxing all affinity's to stagger target prey, occ proc real terror, last 30 sec -1 min, a mini 2 hour stun for drk.

Dark wind = does aoe high level drain/aspir/tp and gives total drained hp/mp/tp to all party members, the more members in party, the more is drained/aspired, leaves a hp down effect on target

Twilight hour = creates a aoe area around Drk that is 50" wide where evryone gets effect of nighttime ( normally 20-04 ) boosts all dark magic used under effect by 25-50% ( including ws based on dark ).
Posible added effect : boosted effect under true night time 20-04, kinda similare to double weather effects as welll as boost on darksday for maxmizing 2 hour.
Gear effects under multiple night time/dark day effects get 2 - 2,5 times the normal boost, ( example smn can greatly lower diablos mp cost as well as increase effect of attacks ) gear that would add +10 evasion nighttime can give +20 or +25evasion with double nighttime/darks day corelation.
Duration 10-15 min

EnRuinra = a more powerfull version of endark that aoe's to party members adding minemum of 100 dmg first hit, builds dark resist down on enemy's hit with wepons effected with enruinra, increasing add.dmg done evry hit
Last 2-3min , also slowly adds +5 dmg evry hit until it caps at 200 base dark dmg on each hit +whatever resist dark down adds.

EnAbsorb = adds enabsorb effect on drk wepon, absorbs random stats evry hit from target and gives to party members/self in 50" range, works superior to regulare bsorbs as effect dont tick down but remain until it wears off.
can take str/vit/dex/agi/int/mnd/chr attk/R.attck/acc/R.acc/evasion/M.evasion/defense/M.defence/critt rate/DA/haste/TA/QA/PDT/MDT but cant drop target under 50% of original stat, ( i guess nm's could be no lower then 75% but no full resists)
perfect for multihit wepon under haste effect to debuff mob's while buffing Party

Zombie team = Turns drk and all in 20" in drk party into zombi ( granting zombi effect , where dmg heals u and heals dmg u )
Last 1-5 min

Diablo's Grace = Grants endrain to party in 50" +dark based refresh giving +4 tick for duration of effect, all gain +150 darknes resist and dark weather 1 effect.
Duration 5-10 min

Colapsing Darknes = Multihit Demi doing 1/4 of enemy hp dmg 2-8 times ( completly random ) once effect is done dark is hit with demi and left with 1/4remaining hp/mp, once 2 hour is used target will gain demi resist for duration of fight.

Ruin Knight = grants access to Ruin spell series for mass casting, also adds quickcast/fastcast to drk
Last 5-10 min , ruin spells start around tier 4 elemental dmg at tier 1 and caps little above tier 5 reg elementh with tier 3 ruin.
Ruin has short recast/cast due to quickcast/fastcast of 2 hour and lowers resist to dark magic on each hit, cost tierr 1 around 120 mp tier 3 220mp max meaning u need refresh+ new ws to keep casting,
can be boosted with dark staff/mab/int etc.