Pros
-Officially replaces Man Eating Bug/Man-Eater Bug  and Tenacious Old Magician/Old Vindictive Magician.
-Recycles any reverse monster, not including himself if it's alone.
-Very combo-able.
-If you have two in your cemetery, you have a infinite Killer Snake/Sinister Serpent.

 

Cons
-Can only destroy 1 monster on your opponent's side of the field.

 

This is one of the most popular cards to come out of the Japanese booster, Pharaoh's Inheritence (309) and the English booster, Ancient Sanctuary (AST).

It's reverse effect is similar to Tenacious Old Magician (Japanese version only) which can only destroy your opponent's monsters.  You might say, "That's not a weakness!"  Well, actually, yes it is. 

 

Being restricted to only destroy opponent monsters reduces the options you get with Man Eating Bug.  Please note that the Japanese version of Old Vindictive Magician only destroys opponent monsters.  In English, it was miss-printed to be almost identical to Man-Eater Bug.  Thank you Upper Deck.

 

Now, that is NOT why this card is so popular.  This card has the most abusive effect in the game.

 

Now, let me clarify a certain ruling first.  When this card is placed (by discarding, by putting, by any way either player can think of) into the cemetery, your to return 1 reverse effect monster into your hand.  Night Assassin CANNOT target itself.  No matter what. 

 

Now, this is where the abuse comes in.  If there is ever a chance where 2 or more Night Assassins are placed into the cemetery, 1 Night Assassin can target another Night Assassin to return all 2 or more back into your hand.  Most of the time, it will come to 2 Night Assassins.  This makes it better and faster then Killer Snake. 

 

Killer Snake's effect only allows it to return to your hand during the standby phase.  Night Assassin's effect will activate AT ANY TIME that it is discarded. 

Exclusive Virus/Tribe Infecting Virus, Angel's Gift/Graceful Charity, Magic Shards Excavation [Not in English], all the XYZ combinations, Armored Dragon... ALL of them will benifit from Night Assassins because of the infinite loop combination of two Night Assassins.

 

Now, one thing I must mention.

 

When this monster flips, it MUST activate its effect unless your opponent has no monster.  When this monster is sent to the cemetery from your hand, you MUST return a reverse/flip effect monster to your hand unless you don't have a legal target (again, not including the Night Assassin itself.)

 

Also note that during your endphase, if you discard Night Assassin, you must return a reverse effect monster to your hand.  MUST... so it's not wise to do it.

 

9.5/10- For a Reverse monster, this card has the strongest potential use in the game.  Works great with any deck that relies on Reverse monsters to do the dirty work.  Killer Snake just got it's @$$ kicked.  Just remember.  This card has the weakness of every defensive monster in the game.  Trample and Emissary of Obliteration/Nobleman of the Crossout will destroy you.  But I don't think of that as a weakness anymore since most of the time, you'd want to keep this in your hand anyways...