Is there any way you can verify that you have completed the event? User Interface in HoTS is terrible. Can't find anything in there.
When you complete the Malthael's Bargain event (on PC), after the 15th match is completed, a screen will pop-up showing "Leoric's Phantom" horse mount unlocked. Once it is unlocked, you can check your mounts inventory to see it (as "owned") / select it for your characters (anytime). The Malthael's Bargain event (listed by clicking the "!" icon bottom middle right of screen, where Special Events are listed too) only appears when you invite a friend to the game and will disappear once the event is completed. Solo games will not display it. So to test it, invite a friend to check if the event is still listed. If listed, it also shows how many games were completed out of 15 before completion. I just completed it playing WD Nazeebo specialist and my friend played Diablo as newbs and leveled to 7. Most were with AI teammates and enemies, but the last few were with other seasoned players (those actually went a lot faster ~ 4-5 mins). Just pick a lane and attack to the core. Game on.
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