Borderlands 2 – How to Start Playing DLC

There is no new legendary items unique to the DLC, only some blue and purple rarity items. And regarding the Seraph crystals, they no longer drop on normal/TVHM, only on UVHM.

Both raid bosses hyperius and master gee have chance to drop variety of legendaries from the main campaign and some seraph items.

Generally the Sandhawk and Rapier are the only worthwhile items on the DLC and they are both easily obtainable by just playing through the dlc. Leviathan's lair has a pretty good chance to drop great relics and mods.

In playthrough 1, outside of the DLC, there's no such thing as scaling. Enemy levels are determined solely by the area they're in and your mission (certain missions effectively "level up" areas, notably Sanctuary and The Dust). You can easily check the level of an area by checking out the vending machines in the area, they sell items scaled to the level.

DLC in playthrough 1 scales to your level...ish. It maxes out at level 30. I played the DLC as a level 34 character and was disappointed to see everything was at level 30 in playthrough 1. Not sure if it has a minimum level it scales to.

Playthrough 2 works the same way until you beat the Final Boss; then everything in that playthrough scales to your level (except mission rewards for missions picked up before the final boss).

Playthrough 3 works the same was as Playthrough 2 after the final boss: Everything scales to your level, period. In team play, it will always be scaled to the highest level player's level. And you have to be at least level 50 to unlock it, so enemies are effectively level 50-61 at all times. Though if you take a mission before leveling up, the mission will still stay at that lower level, as always.

Basically the game works like a standard RPG without level scaling until you A) beat the game twice (because you should be maxed out anyway) or B) enter a DLC (because you can do that at any time so a preset level would be a really bad idea)