Probably the most underused guild ability (in my opinion) would have to be Dimir's Cipher effect which is the equivalent to populate or battalion. Essentially, you cast a sorcery and after it resolves it exiles and encodes itself on a creature, and then whenever that creature deals combat damage you may cast the spell again for free. This would be a good ability with a guild that focuses on damage and creatures but Dimir focuses more on mill-to-death and so because of the awkward fit the ability sees little play in standard. However, the idea to utilize Cipher in this deck where Detain is a common asset (put fourth by flyguy) was one that may or may not make use of the raw potential Cipher has. During a casual gamenight with friends last night I decided to try some various Cipher cards (only ever two at a time which surprised me at how consistently I was able to get them) and boy did they do work. I played a Selesnya health, a Boros aggro, a Golgari control, an Izzet aggro, and a Naya token throughout the night and I only lost once maybe twice. The two cipher cards I decided to go with permanently are Voidwalk and Midnight Recovery.
Midnight Recovery is a sorcery that within itself is a good card: tap four and put a creature from your grave back to hand. Now imagine having the option to do this every turn for free. In the scenario from last night that really stuck out to me I was playing Izzet aggro and the guy kept pummeling me with massive Nivmagus Elementals. I defended as many turns as I could with any creatures out there like Tithe Drinker and Crypt Ghast. I managed to survive to late game where I brought out my Archon of the Triumvirate and waited a turn; I took enough damage on the next turn by not blocking his two Nivmagus that I almost died. When my turn came around again I casted Midnight Recovery and ciphered it onto Archon. So now, I was consistently doing 4 damage in the air while shutting down his massive creatures and every time I did I was getting a low cost creature back from the grave which then re-emerged back onto the battlefield. I went from clutching onto 2 life to destroying my friend with an army of once dead creatures. With its overall consistency and utility, Midnight Recovery is the new black cipher card for this deck.
The blue one I chose requires a little more thinking. This is the spell Voidwalk which exiles a creature then returns it at the end step. Now alone, this card doesn't do that well but my favorite combo of the night was based around this card. Basically, I brought out a Tithe Drinker and next turn attached Holy Mantle onto her making her a 4/3 with lifelink, pro-creature, and extort. Next turn Obzedat, Ghost Council. Now the turn after that when Obzedat returned to the field I gained 2 life and my opponent lost 2. I then casted Voidwalk onto Obzedat and exiled him while ciphering the card onto Tithe and extorted for gain 1 opponent loses 1. I then hit with Tithe for gain 4 opponent loses 4 and then Obzedat appeared during the end step just to be exiled again using his ability but not before giving me 2 and my opponent losing 2. If we do the math here then I now had a factory gain of 9 life per turn from Obzedat, Tithe lifelink, and cipher extort, while dealing 9 to any one opponent. It was a truly beautiful combo. Voidwalk also served utility as being a great way to weaken token decks and also was used several times to exile a Lyev Skynight after attacking just so he could be brought back at end of turn to use his detain again. Overall, [[Voidwalk probably will not find a home in a competitive Dimir deck but it has so much potential in this particular Esper control.
Also, just on a sidenote I've been beginning to question the utility of Dramatic Rescue in a control deck but I was horribly mistaken. I underestimated being able to bounce tokens out of existence, being able to reset a buffed creature, and saving my own creatures or reusing their ETB effects all while gaining two life. Very, VERY good control card.