Spiral Hell with Drepanum has slow start at 1000TP. With typical scythe WSD gear and moonshade it's around 22k on avg. Entropy with Anguta is around 21.5k, Cross reaper with Redemption has also 22k, Insurgency with Liberator ~23.1, Catastrophe has ~26.5k, Cross Reaper with Anguta ~27k and Torcleaver ~32.5k
So Torcleaver clearly wins as single WS at 1000TP
It keeps winning with more TP too, but Spiral Hell with Drepanum has a very solid jump at 2k and 3k TP.
2000 TP:
Entropy Anguta - ~32.7k
Cross Redemption - ~37.5k
Insurgency Lib - ~40.9k
Cross Anguta - ~44.6k
Torc Caladbolg - ~48.2k
3000 TP:
Entropy Anguta - ~35.8k
Cross Redemption - ~52.8k
Cross Anguta - ~49.4k
Insurgency Lib - ~54.3k
Torc Caladbolg - ~59.2k
But ofc Torcleaver is for Light SC and only 2 step. It also has way less accuracy.
Going to 3-4 multistep double darkness or Umbra we have few options
1. Cross/Spiral/Quietus > Cata/Entropy > Cata/Quietus/Entropy
2. Insurgency > Entropy/Cata > Spiral/Cross/Quietus > Quietus/Cata/Entropy
3. Entropy/Cata > Cross/Spiral/Quietus > Entropy/Cata/Quietus
Looking at this I don't find Drepanum good for multistep, unless maybe for one SC per minute. The only WS that takes advantage of it is Spiral Hell and its strong only at 2000+ TP and you can take advantage of it's skillchain bonus only with consume mana, that has 1min recast and adds 1sec JA delay.
In 3 step you can start with strong Spiral, but Entropy and Quietus will both be weak. Apocalypse can do good/strong Cross, then pretty decent 2x Cata or you can start with Cata and do Cross as 2nd WS if you are sure that you can overflow TP on Cross. Either way it should be much better than Drepanum. Entropy > Cross > Entropy with Anguta should easily be much better too and if you start with 2000+ TP Entropy you will make Umbra. Assuming you have AM3 on, Liberator should also be better with Entropy > Cross > Quietus.
Now 4 step is even more brutal to Drepanum. Both Anguta and Liberator can start with stronger Insurgency (Anguta with TP bonus, so assuming you don't start at really high TP). Both Anguta and Liberator will have much stronger Entropy too. Then Cross from Anguta and Liberator (should be possible to overflow a lot more TP with AM3 up and AM3 proc on WS also helps) should easily beat Spiral from Drepanum. Lastly Quietus from Liberator would be stronger than from Drepanum and Anguta would make Umbra with Entropy, so would easily win too. I think Insurgency > Cata > Cross > Cata with Apocalypse would be better than anything from Drepanum too.
What makes Anguta really great here too is that it uses multihit WS, so the chance for breaking SC with miss is very low. Now doing Spiral and Quietus with Drepanum has much higher risk for misses.
Lastly we should talk about Lycurgos with enough HP to get TP bonus +1000 and it's Upheaval > Steel Cyclone > Upheaval > Steel Cyclone
That's only 4 step Darkness, not double Darkness, but it uses only really strong WSs. Upheaval with +1250 TP bonus is as strong or stronger than Torcleaver, tho accuracy (especially for hits other than 1st) might be an issue here. Misses on Steel Cyclone can also easily break SC.