Disclaimer: This guide was made only for reference you can build what you like on your character.
This is original guide and not copied from other sites and made applicable on Ragnarok Forever only or any server that has same configurations if there's any.

Assassin are masters of close combat. They are known to deliver fast strikes with high chance of critical hit.
However this build will focus on damage dealt per hit and disregard atkspeed (Without doppel card). However this build's potential will only work on it's best if equipped properly. This is an expensive build because you need variety set of weapons so it's not recommended for starters. But if they are properly equipped they can be really devastating. They can also switch from dual dagger to Katar Sonic Blow Type with EDP = /gg.

Dagger Assassin stats:

Str: 120 total + bonus
Agi: 80-90
Vit: 20-30+
Dex: 50-60 total + bonus
Int: 1
Luk 1.

Skill Build:

Double Attack 10 = (You can still use this when you changed to assassin even with dual dagger and it gives passive bonus katar damage so you can still switch to katar and deliver sonic blow skill).
Increase Dodge = 10
Steal = 5-10 (You will rarely use this skill get is as hide prerequisite).
Hide = 3 (Cloaking prerequisite).
Envenom = 10
Detoxify = 1

Left Hand Mastery 5
Right Hand Mastery 5
Katar Mastery = 10
Sonic Blow = 10
Cloaking = 10
Enchant Poison = 10
Advanced Katar Research = 5
Enchant Deadly Poison = 5
Create Deadly Poison = 1



Katar Sonic Blow:
Infiltrator [1] Hydra Card, Skel Worker Card, Mummy Card, Phreeoni.
Katar of Frozen Icicle [3] 3x Vadon For Fire propery monsters.
Katar of Piercing Wind [3] 3x Drainliar For Water propery monsters.
Katar of Thornbush [3] 3x Mandragora For Wind property monsters.
Katar of Raging Blaze [3] 3x Kaho For Earth property monsters.
Katar of Raging Blaze [3] 2x Hydra 1x Minorous, For Anubis, orc hero and orc lord MVP. Replace Minorous with Skel worker for PvP.
Katar of Raging Blaze [3] 3x Metaller, The silencer
Katar of Raging Blaze [3] 3x Skel Worker For GH prison and louyang dungeon.
Any [3] Katar with 2x Hydra 1x Skel Worker

Now this part is a bit confusing but I will try to make it as easy as I can. Cards can work in both right and left hand, Elements only work on right hand and Your left hand can only deliver 80% of the damage.
So Basically Your right hand should have either elemental weapon or + attack cards.

Weapon for right hand:
Gladius [3] 3x andre cards.
Elemental damascus (You can forge in this server even without a black smith just bring item requirements in forger npc located at the center of prontera.
Ice Pick
Ice Pick [1] with doppelganger for faster atk speed.

Weapon for left hand:
The only weapon that you need in left hand is +10 Main Gauche [4] unless you want to experiment and waste your time.
Now for the cards you should always combine 2x size modifier , 2x element modifier and 2x race

Here are the cards that you might consider: (It's not complete you can search in ratemyserver for other cards.)

Drainliar card. - Water Property
Anacondaq Card - Poison Property
Kaho card - Earth Property
Mandragora card - Wind Property
Vadon card - Fire Property
Orc Skeleton card - Holy
Hydra card. - Demi-Human
Goblin Card - Brute
Caramel card - Insect
Flora card - Fish
PecoPecoEgg card - Formless
Petite card - Dragon
Scorpion card - Plant
Strouf card - Demon
Desert Wolf card - Small
Minorous card - Large
Skel Worker card. - Medium
Orc Lady Card - Orcs

Upper Head:
Evil Snake Lord Hat [1]
Hunter's Cap [1]
Ghost Bandana
Marionette Doll [1]
Majestic Goat [1]
Poo Poo hat
Helm of Orc Hero

Middle Head:
Evil/Angel Wing Ears
Human Blinker [1]
Blank Ears [1]
Robo Eye [1]

Recommended Cards for headgears:
Deviruchi , Cramp Card + Tarou Accessory

Lower Head:
Pirate Dagger
Gangster Scarf
Anything if you can't afford to create those yet.

Thief Clothes [1]
Pantie [1]
List of Cards recommended:
Porcellio (Atk +25 but it lower your Def -5, Good for agi.)
Peco Peco
Thief Bug

Manteau of Highlight [1] (Dragonfly Card + Chonchon card on Shoes)
Mocking Manteau [1] (Whisper Card)
Immune Manteau [1] (Raydric Card)
Deluxe Manteau [1] (Nine Tail Card) Needs to be +9 or 10
Manteau can be replaced by Undershirt[1] for Pantie+Undershirt Combo

List of Cards recommended:
Male Thief Bug

Ring [1]
Brooch [1]
Gloves [1]
List of Cards recommended:

Leveling Areas:
1-10: (Novice) south of prontera whack everything especially pupae. or go east after south and kill rockers and poporings (faster)
20-40: Byalan Dungeon 1-2 Best with wind dagger, Desert wolves south of Prontera.
41-60: You can stay in Byalan Dungeon 3-4 or Go kill hodes and sandman in south of morroc Assassin's Guild.
61-80: If you have proper equips you can go east+east of comodo and kill Alligatos and Anolians Anubis in Sphinx, Glast Heim Prison.
80-99: Anubis.
That's it. As what I have said this guide is originally made and you can build or follow at your own will.