What are some good mods for an assassin character? Preferably something like new weapons, traps, or spells?

Sneak tools

http://skyrim.nexusmods.com/mods/19447

Daggercraft

http://skyrim.nexusmods.com/mods/6494

Dread knight bow

http://skyrim.nexusmods.com/mods/15494

Dread knight arrows

http://skyrim.nexusmods.com/mods/15799

Black tower and black sacrament armor pack

http://skyrim.nexusmods.com/mods/5696

Masters of death armor

http://skyrim.nexusmods.com/mods/9752

The dance of death

http://skyrim.nexusmods.com/mods/10906

Also look at this thread for random armor recommendations

http://www.gamesas.com/topic/1391726-somebody-recommend-me-an-armor-mod/

The Dread Knight Arrows are only good until Dawnguard. Then you will be able to make all the arrows you want

Yeh but not with the models and textures of the dread knight arrows. Which match the dread knight now very well