This is a sister piece to the Song of Lemminkäinen. Basically, we have runosongs that describe Lemminkäinen on his own, and Väinämöinen on his own, going to Pohjola and having a sword fight with the exact same lines. As one version mentions they go together, I decided to combine these. However, I does create a narrative break when, in one version Lemminkäinen strikes off the head of a son of the Underworld with his shorter sword when in the other, Väinämöinen's sword is longer and he also does this. Therefore, think of this runosong below as an addition to the Song of Lemminkäinen: the beginning of going to the Underworld is the same.
 Sano poika Pohjolainen: "Mitelläänpäs miekkojamme Katellaanpas kalpojamme! Kummank' on pitempi miekka Kenen kalpa kauheampi?” Sen oli iskijä eellä Jo oli pikkusta pitempi Miekka vanhan Väinämöisen Siitä kuu kärestä paisto Päivä västistä välötti Heponen kärellä seiso Peni putkessa makasi Kasi nauku naulan päässä Kuin sitten lyö limmahutti Listi kuin naurin napoja |
Son Pohjolainen ("of Pohjola") said: "Let's measure our swords Let's check out our blades! Which one of us has the longer sword Whice one the mightier blade?" He was the one to strike first Just a little bit longer was The sword of old Väinämöinen The Moon shone from its blade The Sun glinted from its crossguard A horse sat on its tip A dog lied in the tube A cat meowed in the spot of a nail When he then struck He cut his head like the head of a turnip |
Here is a narrative break, where Lemminkäinen or Väinämöinen strikes off the head of the son of the Underworld (happens in the Song of Lemminkäinen). Either because of this, or because his sister (a maiden of the Underworld) is being wed away (to Lemminkäinen I suppose), this son in question wants to enact revenge:
 Se Pohjolan pitkä poika Teki nuolen kaksi kannan Pääskyn pienistä sulista Kahto illon, kahto aamun Tulovaksi Väinämöistä Saavaksi Tapiolaista Kahto suurelle merelle Ulapalle aukealle Pohjolan kujan ovella Ruunun jousta jännittää Väinämöisen souvellessa Ampu yhen nuoliaan Ampu vasten varvikkoa Kohen vuorta korkeata Ampu toisen nuoliaan Tahto maa Manalle mennä Alta kuuen yheksättä Alla kuuen kirjokannen Kilpistellen, kalpistellen Kilpisty ihmisen ihoon (Mäni) kautta kainalon vasemman Ampu vanhan Väinämöisen Tuonen mustassa joessa Kulki kuusissa hakona Petäjäissä pölkyn päänä Oli kolme vuorokautta Meressä, sielt’ sitten pääsi Pohjolan aitain perille Ja siellä sitten itkennä Sen Pohjan emäntä kuuli: ”Ei ole lapsen itkua Eikä vaimon valitusta Vaan on itkua urohon Jouhikuonon jorrotusta” Sitte se oli sen korjanna pirttiinsä ja antanna ruokaa, ja siitä oli sitte päässyt taas. |
That tall son of Pohjola Made an arrow with two bases From the feathers of a small swallow Looked for the evening, looked for the morning For Väinämöinen to arrive To get Tapiolainen ("of Tapiola") Looked towards the large sea To open waters On the door of the alley of Pohjola Drew the arrow of the crown As Väinämöinen was rowing Shot one of his arrows Against the brushes Towards a tall mountain Shot a second arrow Earth almost collapsed to Mana From below eighty-six From below six iridescent lids Rebounded, recoiled Rebounded into the the skin of a human (Went) through his left armpit Shot old Väinämöinen In the black river of Tuoni The went in the spruces as a log In the pines as a block of wood For three days He was in the sea, then got out of there To the ends of the fences of Pohjola And then cried there The Lady of Pohja heard this: "This is not the crying of a child Or the whining of a woman It is the crying of a hero Howling of a bearded-one" She then took him into her house and gave him food, after which he was good to go again. |
Despite having issues with the son of the Underworld, Väinämöinen has no issues with Louhi, unlike Lemminkäinen who has issues with both.
Depending on how you want to look at it, either Väinämöinen married the sister of this "son Pohjolainen" as a result of the competitive runosinging, making son Pohjolainen the same as Joukahainen. Or not, and Joukahainen is the brother of Väinämöinen, and this Pohjolainen is a separate character upset with Väinämöinen. I don't think there is one true answer as oral tradition tends to ebb and flow.