Why?
The Celestial Stone/Portal is a popular feature; many people try to use it when it opens up. As we've seen, MS are becoming increasingly difficult to gain given that in order to get AC, we need to spend MS. This is just one small way to ensure a features continued use, and allow people the chance (not a guarantee, the other possible rewards remain!) to get more MS. Not only that, but it will be towards the tail-end of the event when people are scrambling to get as many MS as they can.

How many MS though?
Similar to the other rewards (30 SB, 30 Energy, 3% Impression, etc), make it low enough that you're not getting inundated, but high enough so that it's worth it. I'd say 3-5 shards. If there's a 1/5 chance of getting it, the approximate amount you'd get from the murderstone celestial portal would be 24-50/day and that's only if you happen to be lucky enough to get it.

Thoughts? If you disagree please let me know! To be honest I'd prefer to trade the impression reward for an MS reward, but I figure just adding MS is probably the fairest option.

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Support MS being a Celestial Portal Reward! https://www.lioden.com/topic.php?id=304429432115&page=1