This set all drops in the instance The Deadmines in Westfall. This is a set worn by many level 19 twinks.

For Horde twinks, it can be tough to get to the instance. I recommend traveling to STV and swimming upward. Just watch out for Alliance as you near the instance.

there is also the cloak of the brotherhood (https://vanillawowdb.com/?item=5193 https://vanillawowdb.com/?item=5193 ) and the bandit shoulders (https://vanillawowdb.com/?item=10405 https://vanillawowdb.com/?item=10405 ) to make a really complete set.

2 Expertise Rating gives 3 Expertise at level 20 which is pretty good.

well, the set is BoE except for the chest. Embrace of the viper is largely BoP.

The interesting thing about this is that it makes it viable for twinking, if you put an armor kit on the leggings, you can still take advantage of the set bonus value... which I THINK supplies better AP for melee, and only slightly less stam then the embrace set. It's pretty good, all in all.

I need to look at the numbers again, though.

Edit:
ok, comparison of typical twinking items vs. defias leather.

https://vanillawowdb.com/?item=10399 https://vanillawowdb.com/?item=10399  standard.
Unless you're alliance, this is the standard item for rogues. SOME will go for the tunic of westfall for the agi (https://vanillawowdb.com/?item=2041 https://vanillawowdb.com/?item=2041 ) , but this doesn't really hurt you.
https://vanillawowdb.com/?item=10403 https://vanillawowdb.com/?item=10403  vs. https://vanillawowdb.com/?item=6468 https://vanillawowdb.com/?item=6468
18 AP vs. 5 agi and 6 stam. I think the new version of the defias belt may be BETTER, but you lose dodge.
https://vanillawowdb.com/?item=10402 https://vanillawowdb.com/?item=10402   vs. https://vanillawowdb.com/?item=19969 https://vanillawowdb.com/?item=19969  or https://vanillawowdb.com/?item=1121 https://vanillawowdb.com/?item=1121
The boots hurt you probably the worst. You either lose out on 8 AP and 5 agi worth of dodge, or you miss out on 10 stam and a BC enchant.
https://vanillawowdb.com/?item=10401 https://vanillawowdb.com/?item=10401  vs. https://vanillawowdb.com/?item=10413 https://vanillawowdb.com/?item=10413  or https://vanillawowdb.com/?item=6586 https://vanillawowdb.com/?item=6586  or even https://vanillawowdb.com/?item=7284 https://vanillawowdb.com/?item=7284
The gloves also lose you between 1 and 8 AP, including between 4-6 agi worth of dodge. They contain 1 stam, as opposed to the 4 in a scouting glove of the monkey. The red whelp gloves actually give a proc that's equivalent to 1-1.3 dps per minute with 1.5-1.9 speed daggers, the equivalent to about 14-20 AP. Being a 5% chance to proc, though, and the fact that a ton of rogues use shadowfang/cruel barb at this level, (and with shadowfang's relatively slow speed for a 1h), no one really uses the red whelp gloves.
and lastly https://vanillawowdb.com/?item=10400 https://vanillawowdb.com/?item=10400 vs. https://vanillawowdb.com/?item=5961 https://vanillawowdb.com/?item=5961 you lose 2 AP and 5 agi worth of dodge.

The set bonuses, however, make up for this by the expertise and AP bonuses.

The arcane resistance offers a slight buff to resisting polymorph, which is nice, but not dependable enough to be consequential unless stacked with more. The armor makes up for a little bit of lost agi, but again, it's not that great a bonus.

From typical twink gear, you're going to be down about 100-200 hp or 4-8 AP, (depending on what you pick) you'll also lose around 10% dodge from the agi. In return, YOUR attacks will gain about 0.95% chance to not miss, not be dodged, not be blocked, and not be parried. for warriors, pallies, shammies, and pets, that's 3.8% more on the table. for hunters and rogues that's 1.9% more. (correct me if my figures are wrong)

druids you'll likely be hitting from behind anyway. =P

Regardless, it doesn't look like a bad option, by any means. and it's more stylish.

This set barely gives any good agility compared the the Embrace of the Viper set. Not very good, but looks a lot better.

For those of you who are comparing the https://vanillawowdb.com/?itemset=162 Embrace of the Viper set with the https://vanillawowdb.com/?itemset=161 Defias Leather set, at the time of this writing...

https://vanillawowdb.com/?itemset=161 Defias Leather Set (complete)

314 Armor
+10 Strength
+9 Agility
+14 Stamina
+28 Attack Power
+5 Arcane Resist
+2 Expertise

https://vanillawowdb.com/?itemset=162 Embrace of the Viper (complete)

333 Armor
+18 Strength
+26 Agility
+24 Stamina
+7 Nature Spell Power
+4 Expertise
+6 Spell Power
+10 Intellect

Those are the base stats without the combined effects. Since the combined effects will vary depending on your class (and talents in some cases) I leave it to you to do the math and decide which is best for your character.

https://vanillawowdb.com/?compare=6473:10412:10411:10413:10410;10399:10403:10402:10401:10400 Here are the maths!

Embrace of the Viper (with set bonuses):

+17 Agility
+10 Stamina
+8 Strength
+19 Armor
+2 Expertise
+10 Intellect
+6 Spell Power
+7 Nature Spell Power

Defias Leather (with set bonuses):

+28 Attack Power
+5 Arcane Resistance

The 17 Agility and 8 Strength totally make up for the 28 Attack Power of the Defias Leather.