#rennala had TWO toxic betrayal-based lesbian relationships. at LEAST two.

[snarp]

Once she's done telling you the above lies, Sellen hands you this:

(This thing's skill's animation looks like a sneak attack/backstab.)

The specific nature of the symbolism of the "undulating blade" is explained by Jerren's sword:

(This one's default skill's animation is "attacking something looming over you.")

They both symbolize starting fires - which is, itself, treason - but the Kris very specifically says "setting the Erdtree on fire and betraying you." Rennala gave this thing to Marika for the same reason Sellen gives it to the player.