Nemesis league

"Rare monsters have one guaranteed mod from the Nemesis Pool, which makes the fight substantially harder."

- Description on the official Path of Exile website

The Nemesis league was a four month challenge league aimed at players who are looking for more challenging gameplay. In this league, every rare monster had one powerful Nemesis modifier, making them even more deadly.

It started, along with the Domination league, on October 23, 2013 and lasted until February 23, 2014. At the end of the league surviving characters and stashes were moved to the Hardcore league. As with regular Hardcore, characters that die during the four months were moved to the Standard league immediately.

Modifier
All areas had the following modifier:

Items
Nemesis had seven exclusive base item types (all shared with Domination) and seven exclusive uniques (six of which were shared with Domination). The new base ring type did not drop from slain enemies, and instead was only obtainable by using a new vendor recipe. The gem could not be obtained, and IIQ mods could not spawn on magic and rare items.

Exclusive base item types
The following item types were introduced exclusive to Domination and Nemesis. Following the end of the leagues, these base types became available in standard and hardcore leagues.

Four flask types:

And three ring types:

Exclusive unique
The following unique were shared between Domination and Nemesis leagues as league-specific items. Since version 3.14.0 they are added to the core drop table (while is always a vendor recipe only item) but before that, already obtainable from divination cards and beastcrafting recipes:

The following unique flask is Domination and Nemesis leagues exclusive. After the league end it normally requires Zana crafted map mod or other way that able to grant league-specific item, such as beastcrafting recipe and divination cards.

This unique was specific to Nemesis leagues only. Since version 3.14.0 it is added to the core drop table: