I honestly don't find I use Relentless much. I usually prefer to have my rotations more tightly controlled, and having extra turns messes that up.

That being said, since it's low probability, it's best used in places where you have a lot of turns to go through, so you'd want to focus on champs you're using pretty much exclusively for dungeons or faction wars. Within that set, you're probably best off having it be on champs that have specific skills which are useful to have on low cooldown. Scyl is a common target - every extra turn she gets heals your team, and lets her use her AOE stun more often. Siphi may be a good target as well.

My  siphi  already  has  it  i  was  thinking  about  my  elder  skag  or  my  bellower

Really depends on the stats, you can't just throw a set on someone.... otherwise u end up with a crappy geared champ like the Tower above.  Siphi is an excellent choice if the stats are good.  My guess is the best use is to roll it and improve Siphi.

Go  for  stats-  not  sets.

Work  on  improving  current  builds  first.

And  we  would  need  to  see  the  set  as  well.

how  Would  you  build  him  i  gave  him  what  i  had  at  the  time  im  working  on  my  cupides  at  the  moment  then  i  can  turn  my  time  to  him