Mine says 10x o.o 

You are correct.  That settles that.

I definetly don't go for Theresc. 

There is absolutely no logic in the reduce resitance debuff, unless this debuff itself would be irresistable like Ghostborn's decrease def. Where could you need decrease res at all? Only vs. enemies that have very high resistance. But as this debuff is not irresistable itself, Theresc needs enough accuracy to place it the old fashioned way.

Conclusion: You have to build one champ with enough accuracy to beat their resistance without that debuff active (Theresc beeing that champ). 

But if you have to build a champ with enough accuracy to beat their res, why do you need Theresc at all? Just put your main debuffer in the high acc gear you would put on Theresc otherwise and this champ should land his debuff without decrease res active.

That Theresc has only a booked 75% chance of landig this debuff comes on top of that general problem. Vault guardian epic for 40 (!) ancients.

i went for her.

odds are i shelled out 40 shards for a vault guardian, but they were shards i could afford to lose. she definitely seems underwhelming right now, but i wouldn't put it past plarium to pull another head of mischief and design new content to make decrease resist more viable. i'd much rather risk wasting some resources than find myself six months down the road desperately wishing i had gotten her when i had the chance.

that said, i do think she seems rather useless and i wouldn't have bothered if i hadn't been sitting on about 200 ancient shards.

You make very good points.

I went for her, but only because I try to get all these nerd champs to test out for other people. 

The  thing  is,  decrease  res  has  just  been  added  to  the  game.  Her  one  skill  with  it  really  sucks,  and  the  rest  of  her  kit  really  sucks.

Remember  that  one  rare,  Kurzad,  who  had  Increase  Acc?  Well,  shortly  after  that  they  added  more  champs  with  it,  and  ones  that  were  much  better.

I'm  expecting  the  same  thing  to  happen  here.

I'm expecting test results after CvC, or else. 

I would have liked to see her have the decrease res to land before the AOE A3, at the very least, to proc the passive more often. I think it should be 100% when booked, or similar to Ghostborn's dec def as Skadi said. 

SIR YES SIR 

I was unsure if I should go for Theresc or not, eventually I decided not to pull shards for her.

Reason number one is that her signature move A3 has only 75% chance, if booked. In all cases I would like to use Theresc it would be crucial that she consistantly applies her decrease resistance debuff. Therefore, it is a necessity that the chance is 100% if booked otherwise it is not realiable.

The 75% chance reminds me on Ultimate Galeks HP burn abilities. At the time when the Doom Tower was introduced I trained Ultimate Galek because of his HP burn skills. He was the only option for me at that time. I used him for the first couple of months, but I decided quickly that I have to replace him. His HP burn abilities are not realiable enough for the Frost Spider. Now, I do not use him anymore as HP burn champion. I tried to use him for Hydra but without any success.

Due to my past experience with 75% chance debuff champions (Ultimate Galek was not only one) I do know that those champions are not realiable enough. In hindsight leveling Ultimate Galek was not justified taken into account all the ressources I put into him and how often I did use him afterwards as HP burn champion. I am pretty sure the same will apply to Theresc.