My commander and companion have no synergy at all. None. But a Jeskai commander with a Golgari companion is just too clean of a way to do pretend 5-color partners for me to pass up. So the deck is 5 colors, which gives a lot of options, which I need because I can only play lands and creatures, and I refuse to play any creatures (even tokens) that aren't legendary because only legends get banding.
The inspiration of this deck is in the banding lands. Namely, these:
There are, of course, equivalent lands for both black and blue, I currently have the blue one in, and may add the 5th in the future. I will eventually spruce this thread up with a longwinded description of the rules for banding and why banding matters and detailed descriptions about my card choices, but for now I'd just like to give a brief summary of what I've learned playing banding cards.
1) Sometimes banding is awesome. You know when you build a deck based on a stupid gimmick, you usually end up finding out either that it is actually good and not gimmicky enough or not actually good enough to play at all? Banding is neither, sometimes it is awesome. When is it awesome? When you automatically win at combat and nothing can stop you. But that isn't automatic, you have to engineer that scenario.
2) Banding requires aggression. You need to be attacking. Lots of creatures, big creatures, a big commander if possible, haste all over the place, and no hesitance to make enemies right away. Banding lets you attack without worrying about losing important creatures to blocks, so banding works great with powerful triggers that require attacking.
3) Evasion doesn't matter, every other keyword matters a lot. Haste, trample, first strike, vigilance, deathtouch, reach, lifelink, and definitely indestructible... you want creatures that are good attackers and good blockers, preferably both. The downside of banding is that any creature in the band being blocked means the whole band is blocked, but the whole band fighting together lets you take advantage of combat abilities working together: a deathtouch creature can mow down blockers while a trample creature tramples over. One creature being indestructible or immune to damage effectively makes the whole band immune to damage.
And just a couple notes. I have deliberately avoided having non-legendary creatures because they wouldn't be able to band and that stinks. I can't play non-creatures because of Umori. And if I want the deck to instantly turn from ultra-casual to combo machine, I can just substitute Sisay back in as the commander for a game or two until I hate it.