On the League of Legends Wikia I found out that an Altar on the new Twisted Treeline says to Hecarim “Mordekaiser is not to be trusted, Hecarim!“
Why? I can’t find anything in their lore about their past relationship.
In an AMA about the new Shadow Isles patch posted on Reign of Gaming’s web site today, that same exact question was asked:
Q: Why is the left altar saying to hecarim when he caps it, that he should not trust Mordekaiser? […]
A: […] We have developed an epic and comprehensive story for the shadow isles that we hope to fully explore in the future. The interactions at the altar are giving hints at this storyline. I am glad you are curious as to why the spirit is telling Hecarim he should not trust Mord, I am not going to reveal that today but it will become more clear over time.
So that’s all the information available at the moment. We’ll see in the future!