The middle one is by Developer B
Besides its effects, another reason that makes this card so strong is how easy it is to summon it, and its summon is considered equivalent to a link-1 summon by many players
However, this has never panned out, just as stepping into a competitive format with a bad deck and no experience is unlikely to lead to success
The amount of life lost or gained is calculated as the triggered ability resolves based on the creatures you control at that time, including Skemfar Shadowsage itself if it's still under your control
In a dedicated Deck, however, this card is mandatory to run three copies of
If you are going to pick a Sneak Attack then it is best to do it early and then pick up the things to support it rather than the other way round