Arena is free-to-play and great for faster paced Standard and Draft gameplay while MTGO is slower and harder to learn than Arena but great for Modern, Pauper, and generally more competitive players
Combust cannot be countered, but if it is on the stack when Sundial of the Infinites ability resolves, it will be exiled, which isnt countering
Same Set Symbol, Different Border Unlike newer reprint sets, Chronicles cards are reprinted with their original set symbol, but with a white border
Sacrificing no longer means going down in material, and the other players usually struggle to keep anything on the board at all
You also cant make any choices or decisions for the player that would be called for by the tournament rules (such as whether to take an intentional draw or whether to call a judge)