More than a dozen mages are getting reworks, ranging from large-scale changes to characters like Zyra and Brand to smaller but significant shifts for Xerath and Ziggs.Dragon spawns are changing to include a new set of elemental dragons, replaced 35 minutes in by an Elder Dragon that offers big rewards for taking him down. Combine that with the changes to existing magical items, and players can expect to wind up with a very different game in the wake of the mid-season shifts.
Source: Official Site