So what does the Legend number actually represent? Also, I’ve noticed that over time if you don’t play, that number decays: You always wake up a worse Legend rank than when you went to bed.
What exactly is the rate of decay? Is it not as much decay as it is other players taking your spot? Does it tend to be worse the lower you are? For example, would a rank 300 Legend decay faster than a rank 9,000?
Im not thoroughly educated how this works, but I assume that it drops more on lower ranks as new players enter legend or move past you. Theres less movement on higher end of ladder and I dont experience big drops in top 300. I havent read any articles how it really works but I assume the higher your mmr is the better rank you get when you enter legend for example.
edit. maybe somebody whos read something about it should answer, i just assume its all mmr based 
its your individual rank, there is only one rank 3001 for example so for every player that climbs past you your individual rank will drop because someone else above you now has your old 3001 rank and so on. how much you climb and drop depends mainly on your mmr but also the rank of your opponent. at high legend you climb slower and drop slower, but you drop more if you lose to a rank 500 compared to a rank 10.
Does it matter when in the month you get Legend? Let me give an example:
-June 25th I reached Legend and was a rating of 1350. (It was very easy to climb in June. I hadn’t played in Wild in a very long time and many of my opponents didn’t seem to have meta decks. Some just piloted them wrong)
-July 7th (today) I reached Legend and am a rating of 385. (It was very difficult this month to climb. Perfect decks/plays from my opponents. I was stuck hovering between Dia2 and Dia1 for like 3 days)
My guess would be that MMR doesn’t change daily, but updates at the end of the month.
MMR do indeed change after every match. the difference this month is that your starting mmr is the same as your ending mmr from last month and since you had a high rank you also had a high mmr and got matched against other players with high mmr thus making the climb more difficult because your opponent were jsut better than last climb. as your first legend rank also shows your mmr was indeed high.
I’ve been curious about this too, as i’ve played a bit of wild legend too and got up to 50, beat a couple of #9 players, had it barely nudge up a few points like 5-10 when beating them, but a single loss makes it fall like 20-30 easily. Then there were a couple of times where i fought like Diamond 3 players who had strong decks but just aggro, and gained more than losing/winning to the higher ranked legend players. One time i even ran into a game where we were both the same rank against eachother like rank #261 vs #261 or something a month back. I can’t remember the exact number but they were both the exact same number, not 1 off, but identical. both of us marveled at it a bit but went on. I think it is a player number for the most part, but who knows im not 100% sure if the system ties you on mmr if you’re supposed to share a rank number though or if it was just lag.
this happends sometimes when your opponent gain your rank just before you are matched, your visuals hasnt updated yet with your new rank when the game starts while you actually are one rank less.
How can I see my mmr?
you cant. unlike overwatch hearthstone dont show the mmr value