Whats the process of creating a new champion as a designer? Are you creating them from scratch, or do you take a mechanic/concept piece and expand on that?

Since I've been getting a bunch of questions about this, I'll try to give a rundown of what it's like when we create a champion here at Riot.
We start with a process we call ideation. We generally enter ideation only with a vague idea of what kind of champion we want to make (for Tahm, this was "tank") and after a month or two have it drilled down to one concept that excites us all. This includes many concept sketches, mood boards, story sketches, paper kits and a playable early prototype in game Frankensteined together out of models/animations/sounds/visual effects from other champions.
Once we know enough about the champion we're making, we're going from ideation to pre-production. This is where the group of people making the champion (which we refer to as a pod) starts growing. At this point, we make a proxy model (a very rough 3D model of what we think the champion should look like) with some placeholder animations. We generally have a "play pattern" at this point (think "Yasuo dashes through minions, stabs with Q, and every third Q shoots a tornado"). We like to have 3/5 abilities locked down by the time we leave pre-prod (we consider the passive an ability)
After we've nailed down the champion to this point, we spin up into full production. At this point we bring the full pod online. Next to a gameplay designer, a narrative designer, and an artist (the "DNA" of the champ who are there from the first day of ideation), this now includes producers, dev managers, animators, 3d artists, texture artists, visual effects artists, sound engineers, riggers, quality assurance people, the entire promo team (the people who make the spotlight, the people who make promo assets like Ekko's "Seconds" video and the Ekko comic), and live designers and dedicated playtesters to help with balance and overall game health. Phew. That's a lot of people!
Once we're "done" with the champion, that is to say once all assets have been created and polished, we go into post-production. This is where we rush in some last minute stuff, try to fix as many bugs as we can, do last minute play pattern adjustments (Tahm's ally devour used to only give move speed TOWARD the enemy until the playtest team pointed out that Tahm support loses lane HARD once he starts losing because he has no fallback pattern; that's why I added half MS when not moving toward the enemy at the last minute in post-production, and that's why it's not in the spotlight). During post-production, a champion goes to PBE as well.
And then we're live! This whole process, from first ideation meeting to the champion being purchasable in store takes about 9 months currently, but can take considerably longer on some champs, particularly if we run into questions that are hard to answer.