Vampire

Like lycanthropy, the player can willingly contract vampirism and become a Vampire. Doing so will grant them additional skills.

How to become a Vampire
There are two ways to become a Vampire in the game, one is costly, the other probably very time consuming. Once players with vampirism have reached level 6 in the vampire skill line, and have acquired the passive skill Blood Ritual can turn other players once a week. Given, these are willing to take the "gift". Of course, most players will demand payment for this treat, which easily can reach 10.000 and more. Much more.
 * Advisory note: To avoid scam, never pay in advance.

The other way very much obviously depends on the right moon phase. There are rumours that the infectious Vampires spawn at a special moon phase and probably only on certain days, perhaps only every 8 days. Also, not every bite of a Vampire will turn the player into one. There are special Bloodfiends, which spawn in certain locations of level 37-44 zones, and are carrying the disease. Willing players should look around in Bangkorai, The Rift and Reaper’s March to acquire vampirism. One also should not be too weak, as the bite drains much energy.
 * Advisory note: Try not to kill the Bloodfiends, as other players might get upset.

When the deed is done, one can find the Vampire Vorundil outside of Riften, who will clarify the situation and also offer the level 38 quest Scion of the Blood Matron, which finally will grant the Vampire skill line after finishing a ritual. Vorundil will describe the options and either send the player to Rahaja, another Vampire, or to a priest of Arkay, in case the player demands a cure. For other factions, the contact person of course appears somewhere else.

Bloodfiend locations in the Rift
It seems the Bloodfiends mostly spawn near certain wayshrines south-west, south-east (close to Rifton) and north-east.

The downside
Being a Vampire requires the player to feed on the blood of creatures. This is necessary very periodically, or the skills will suffer. Drinking blood can be achieved in sneaking up from behind the prey, and at some point one can start the action in pressing X. When more skills open up in the process, there is also the possibility to turn into mist, which makes this skill line rather interesting for Nightblades. But more on the downsides, as the disease has several stages (like in Skyrim):

The complexion will change extremely during this, until one looks very ill and pallid with protruding veins and horrifying eyes. When one is in stage three or four, it isn't advised to enter any area with firemages, daedra or similar. It's a very hard task to keep health levels up long enough then or to regenerate.
 * Also note: Fighter's guild skills cause higher damage, so take care whom you meet in the PvP area!