That is a combo that is about to be legal in standard. Is wizards finally relenting on combo in standard and letting us have it? They did print Ugin's Nexus after all.

EDIT: I should explain the combo. [[Sage of Hours]] has an activated ability of removing all 1/ 1 counters from it. If you remove five or more, take an extra turn. [[Roaring Primadox]] bounces a creature every upkeep. The newest spoiler from Fate Reforged puts five 1/ 1 counters on your lowest toughness creature.

Step 1: Have Sage of Hours and Roaring Primadox on battlefield

Step 2: Cast Mastodon, put five counters on Sage.

Step 3: Remove counters from Sage.

Step 4: on extra turn, Primadox bounces Mastodon, and you repeat.

Infinite turns!