I think there's a really good case to be made for which dagger would be preferable to use between this and the Corvian Greatknife, as a general purpose sidearm. There's three factors: critical damage following parries/guard breaks, against shields, and as a stand-alone weapon. I've tested them both +10 Lightning-infused with a 26 STR 40 DEX 30 FAITH build. For critical damage, Mail Breaker (125) takes it over Corvian Greatknife (110), but not by much, as the Mail Breaker's crit multiplier is better, but it's damage is lower; against Black Knights NG+1, Mail Breaker did 956 w/o Hornet (1187 w/ Hornet) while Corvian Greatknife did 877 w/o Hornet (1080 w/ Hornet), which is a ~10% difference. Against shields, both have WA's that avoid shields, but I found the Corvian Greatknife vastly superior for a few reasons: Blind Spot hits around their shields for 270 whereas Shield Splitter hits for 157 and Blind Spot is a much better approach as it's much faster and less telegraphed. Both daggers bounce off higher stability shields (which also gives an edge to Corvian, as starting off with Blind Spot makes an opponent much less likely to sit behind a shield, letting you R1 mash once they let it drop, whereas they'll roll the more obvious wind-up of Shield Splitter and put their shield up again) but can be spammed against lower stability ones (I haven't figured out exactly how this works, but they can chew through the Dragonhead Shields of Ringed Knights but not the Black Knight Shields of Black Knight, not entirely sure about players). As a stand-alone weapon, there's a fair bit to consider: CGK is 1 unit heavier than MB but has more damage (351 vs 328 2H on this build); CGK has much better range than MB but also has a worse R2 (this weird downward slash), which wouldn't matter too much except MB's R2 (normal thrust) is a fantastic wake-up attack following a riposte which is a major reason to have a dagger; Blind Spot is an excellent starting move and gives you options how to approach (R1 or WA) but not as a roll catcher (and often feels like the Frayed Blade WA-R1, meaning you have to hope they dodge directly back or they'll avoid it), whereas Shield Splitter is an amazing roll catcher as the timing is perfect, the direction of the lunge isn't determined until the last second and so can catch people if they roll to your left or right (but not behind, as it can only hit in a ~180 deg cone in front of you), and it combos into itself (WA-WA is a thing). If I had to weigh in personally, I'd say if you have difficulty getting aggressive and making approaches with your main hand or struggle against shields, than Corvian Greatknife would likely be a better option as it has multiple ways to approach (WA or R1 pokes) and punish, but if you're good with parries and/or are looking to finish opponents off (catching panic rolls of low HP players) then the Mail Breaker is for you. All that said, they're both absolutely excellent, infuse well with Lightning on any build, and do really well with the Leo Ring.