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This item, when attached to a Pokémon that is capable of evolving, will boost the Pokémon's Defense and Special Defense by 50%. Has no effect on Phione or any other Pokemon that is not capable of evolving.
Eviolite is one of those items that has single-handedly made a metric tonne of Pokemon viable. That boost to defensive stats comes at the cost of losing out on Leftovers or boosting items, which is a shame, but on the plus side, some already competently defensive Pokemon can be made nigh-on unkillable. The biggest example of this is, of course, Chansey, whose defensive stats totally eclipse its evolution, Blissey, making it a veritable titan on the specially defensive side and pretty damn bulky on the physical side as well. The downside is, of course, that it lacks Leftovers and attacking moves, so much rely on Softboiled and Seismic Toss, but many would say that this is a price well worth paying.
Of course, that's not to say that there aren't other Pokemon that can use this item as well. Dusclops, like Chansey, is already pretty bulky, and this gets even more rock-solid with Eviolite, enabling Dusclops to not only wall just about anything in the game, but also to irritate the opponent with Will-O-Wisp and Pain Split. However, Dusclops, like Chansey, has to rely on Seismic Toss to be an offensive threat, but unlike Chansey does not have reliable recovery, which can hinder it. Porygon2 is another great choice, who does happen to have access to Recover as well as pretty good offensive ability, backed up by Thunderbolt, Ice Beam, and STAB Tri Attack. All of these combined make Porygon2 an excellent tank which is just as viable in OU as it is in UU. Hippopotas can also make use of it in order to be less of a liability. Further down the tier chain, we see even more bulky NFE Pokemon making use of Eviolite, such as Ferroseed and Munchlax.
This is not to say that primarily offensive Pokemon cannot make use of Eviolite, either. Scyther is fairly frail even by RU standards, but is quite capable of forgoing its Life Orb and running Eviolite alongside Roost in order to create a fairly comprehensive bulky attacker build, or to set up with Swords Dance. Rhydon, too, can use its elevated defensive stats alongside its base 130 Attack, base 105 HP, access to STAB Stone Edge and Earthquake, as well as excellent coverage in Megahorn to wreak havoc. Roselia and Murkrow, too, can put their excellent support movepools to good use while at the same time being able to deal out damage—Murkrow also has access to Prankster to make the deal even sweeter. Various Dragons, such as Zweilous, Shelgon, and Fraxure, are also made more viable—Zweilous's Hustle ability means that its Outrage is more powerful than Rayquaza's (though Rayquaza admittedly has the option of holding a Life Orb), while Fraxure and Shelgon can use their elevated defensive stats to set up Dragon Dance for a sweep, or Shelgon can try a support role with Wish.
Finally, we come to the environment most affected by the presence of Eviolite—Little Cup. With the banning of Berry Juice, Eviolite is the premier item for defensive Pokemon, contested only slightly by Oran Berry, and is really the only reason that defensive teams can exist in Little Cup. Defensive Pokemon like Bronzor, Ferroseed, and Chinchou rely on this item to enable them to wall strong opposing Pokemon such as Drilbur, who at Level 5 are capable of greater damage levels overall than at Level 100. Offensive teams are no strangers to Eviolite either— the added bulk provided can save your bacon in more than one situation, and can more than make up for the loss of power from Life Orb or a Choice item, or the potential healing from Oran Berry. Indeed, it is not out of the imagination for an entire team in Little Cup to all be carrying Eviolite—it really is that good.
Castelia City - Talk to the scientist on the upper floor of the building with Patrat in it, after having seen 25 different Pokemon.