Deep Terror Nami1 votesThe "Keystone Mastery" is just the final ones in each tier, and you can only pick one. Just set up your masteries like you always do.
For replacing the Flask, you would probably want the Corruption Potion; you have enough gold to start with it.
Not sure why you think the masteries are bad. Most of them are stronger, and the Keystones definitely are. You just have less options because you only get 5 or 1 point for each tier.
stealthfox941 votesThere just seems to be a lack of diversity to me. And does Corruption Potion give the same stacks as flask? I need to start it on bad early game champs.