Besides Youmuu and Yi's ult, who else has a AS+MS buff?

Youmuu's go in par with botrk usually. It's a classic build on a bunch of champions such as use. The flatpen increases botrk's dmg and active as+ms goes with the all in nature of those champions.

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topic.

And by extension, who else has Youmuu as core?

passive gives him both AS and MS.

has both in his frozen domain along with increased healing from all sources

Lulu and Nami gangplank blitz eve Janna

Youmuus core, attack speed on empowered q and msp on ult.

Nunus W and Dravens W.

Since you mentioned youmuu's..

Items with AS & MS: (If you count the Active)

Champions with both in a single ability: (sort of, her W gives her AS and also procs her passive MS boost) (technically. Ultimate passively gives her AS while using her Ricochet, and active gives her a MS boost) (stealth gives him MS, and he gets AS when he comes out of stealth) (every time he switches stances he gets MS/AS)