Koschei the deathless is a Fremennik warrior that players must fight in order to complete The Fremennik Trials quest. The player must defeat him three times without dying; although they are told they must defeat him to win, the act of dying to him having defeated him three times is considered enough to win you the vote. This is because the test is one of bravery, not strength.
Koschei's level is not shown; however, he will get progressively stronger and his attacks faster each time he is defeated. His max hits are 3 in his first form, 5 in his second, and 7 in his third. Although the fourth form is difficult, it is not impossible to defeat him; if he is defeated, the player will be rewarded with a Fremennik blade. Koschei's last form has 255 Hitpoints.
A ring of recoil can be an aid in killing the fourth form. When he hits, he always hits for 1 hitpoint, but at a rate of 3-4 hits per second, and due to the rounding-up of the ring of recoil, would always hit him back for 1 at the same attack rate. Note, however, that a player would have to bring several rings of recoil, because each can only reflect 40 Hitpoints.
Low-levelled players may find his third and fourth form rather difficult, so a gnome amulet or combat bracelet is strongly advised, for it will provide a little protection (do not wear a ring of life, or you will most likely have to start over again if your Hitpoints get too low).
There are certain safe spots in Koschei's basement, such as the pillars of fire, or the many chasms in the ground. Players need only run quickly over to one side of a hole, and because Koschei only moves at a walking speed, players should be at one end of a hole long before he reaches the other end. However, do not stay in a safe spot for too long, or Koschei will disappear and you will have to fight him all over again. It is also strongly advised that players bring high-healing food, such as monkfish or sharks, as in his third and fourth form he hits progressively harder and quicker. Also, it appears that if you take too long fighting Koschei, he will simply disappear and you will have to start the challenge all over again. It is interesting to note that, if you manage kill him three times, decide to try and kill him a fourth time, but find yourself unable to, the trial will still be deemed as a success. Once your life points reach 1, the player will kneel down and be transported back to Thorvald, still completing the trial. This is because the challenge is a test of bravery, not strength.
Koschei has many traits (including the trait of being hard to kill) that are similar to that of the Russian legend of the same name. However, in this legend, Koschei's soul was in a needle, inside an egg, inside a duck, inside a hare, inside an iron chest, which was buried under a green oak tree, which is on the island of Buyan, in the ocean. The only way to defeat him was if the egg or the needle was broken, and in some legends it has to be done on Koschei's forehead.
He has a Zamorak symbol on his chest. However, in which way he is, or was, related to the god is unknown, or, if he is just a supporter. However in Runescape 3, it is later revealed that he is one of the Mahjarrat.
Koschei's name is spelled Коще́й Бессме́ртный (Koschéi Bessmértny) in Russia where he is a popular figure in its folklore. As is usual in Russian transliterations, there are numerous other spellings. The spelling in other Slavic languages (like Polish "Kościej", Czech "Kostěj") suggests that his name may be derived from "kost" (rus. кость, pol. kość, cz. kost = bone); thus suggesting a skeleton-like appearance, which also indicates a possible connection with the Mahjarrat.
He is the fastest attacking NPC in the game (in his final form with an attack speed of 9, meaning every 0.6 seconds), along with Scarab swarms.
The attack speed for his first three forms is exactly the same as unarmed combat.
"Koschei" is also the name of "The Master", one of the antagonist in the famous British sci-fi Doctor Who. The Master displays an ability to "regenerate" more number of times than the other members of his species can, hence, in a way, he can also be considered "deathless". The first few "faces" of the Master include a beard and a moustache.