"BerMalBerIst" is the Infinity runeword in Diablo 2: Resurrected. Therefore, you must put them into a 4-socket polearm in the order of a Ber rune, a Mal rune, another Ber rune, and an Ist rune. The most important thing is that you can't change them by the way during the process of putting them into the rune, otherwise it won't work.
It should be noted that you cannot find any type of 4-socket polearm anywhere on normal difficulty. Even if ordinary polearms found on normal difficulty are added with sockets by using the Horadric Cube or Larzuk, there will not be as many as Buy D2 Resurrected Items four sockets in the end. Therefore, if you want to find a 4-socket polearm, you need to play the game on Secret Cow level, Nightmare or higher level of difficulty. In addition, to increase the chance of finding items with sockets, you need to remove all equipment that increases the chance of finding magic items. And no matter where you find a Bardiche, don't add sockets to it, because Bardiche will never have more than three sockets.
However, all the runes in this rune word are difficult to find, so two Ber runes are required, making this rune word particularly difficult to complete. You can find the Mal and Ist runes on Act V on Nightmart difficulty, and then drop them at the Hellforge or The Countess on Hell difficulty. But Ber runes will not drop Diablo 2 Resurrected Items in The Hellforge or The Countess on any difficulty, so you can only put your hope in your luck, so that you can be lucky in Acts III, IV, and V on Hell difficulty. Randomly drop two Ber runes.
In general, Infinity polearm is one of the most difficult items in the game. To find the rune to synthesize it, not only the player needs to have high enough strength, but also the player has enough luck. Therefore, it takes a lot of time for players to make an Infinity polearm in the game. If you want to get it faster, you can come to MMOWTS to have a look. There are various Diablo 2 Resurrected Items for sale, including Infinity runeword. You can buy it directly, you only need to wait 15 minutes before you can use it in the game.