Authenticity guaranteed.
Very large antique russian / caucasian kinjal dagger.  
Probably northern caucasian, dagestan, circa 19th century.
Exceptional quality hand forged fullered sharp blade with gold inlaid arabic calligraphy on one side.
Iron domed rivets cups on front of hilt. 
H0rn grip slabs.
Domestic cattle h0rn (mentioned to comply with ebay policy). 
Cattle is a large domestic farm animal kept for their milk or meat. This animal is not threatened or endangered or protected by cites, the endangered species act, or any other state, federal, or international laws. It is legal to sell this item and this listing follows ebay rules.
Leather over wood scabbard (sheath), definitely working life period replacement. 
Looks like scabbard was "field made", maybe during russian civil war - quality of the scabbard doesn't mutch high quality workmanship of the dagger.
   large size:
About 23 5/16 in. (59.3 cm) long in scabbard.   
About 23 1/4 in. (59 cm) long out of scabbard.
Blade is about 17 1/4 in. (43.8 cm) long and 1 5/8 in. (4.2 cm) wide at widest point.
  condition: 
Very good original antique condition with honest wear and charming indication of age. 
Stable age crack at the front of grip. Handling marks and age patina to hilt.  
Lightly cleaned blade is in very good antique condition with expected age wear, would benefit from more cleaning and polishing. 
Gold inlaid islamic calligraphy on blade is about 90% intact.
Leather on scabbard shows expected age wear. 
Superb, real fighting antique example.