Weave warp (errata)

You attempt to exert your control on an external spell, choose a spell being cast within range, you may attempt to redirect it. When you choose a spell to warp, roll a casting ability check against the spell DC. The DC equals 10 + the spell's level +/- the difference in spell slot levels (EG: a cantrip warped by a 5th level spell would have the DC of 10+0-5, or 5). On a success, you may choose another target within the range of the original spell. This spell fails if there is no other Valid target (the original caster is not a valid target).

(edited to include forgotten caveats)