There are a few modern variants of the valknut, which, however, may be considered valid even though they are not identical to the archaeological forms. Most spectacular of these is a version which combines both the Borromean and unicursal forms into a sort of "Grand Valknut":
However, if one is going purely for accuracy to the surviving history and artifacts, only the first two types of valknuts are historically authentic (and in our time of total reliance on technology, I doubt one can even still find a warrior worthy of the "Arhat's knot" anyway, as strength alone is far from enough).
Consider the Borromean triangles type, which occurs on the Stora Hammar rune stone.
The unicursal type of valknut (which can be drawn with one stroke) appears twice on the Tängelgarda stone:
Here we see Scythian warriors bearing tribute to Darius the Great of Persia at Takht-e-Jamshid ("Persepolis"). Once again, there is a horse and oath-rings, though here the horse is not ridden as it is a gift for King Darius. You can see them wearing similar long cloaks and the same sort of straight Seax-type dagger as in the Viking carving. The name Seax or Sax (Sakhu in eastern Aryan tongues) must be very old indeed, predating the divergence of eastern and western Indo-European peoples, as both the Saxons and the Scythians (Saka) took their name from this weapon. Very few other cultures in history allowed vassal nations the honor of bearing their arms in the presence of the King - and nearly all of them had an Indo-European heritage. Darius was himself a highly successful general with a dizzyingly long military career spanning over thirty nations and kingdoms.
The valknut itself, however, does not appear on artifacts outside of Northern Europe, though again, this may not mean the symbol was unknown to other Indo-European peoples and traditions further south and east. The trifos or triskelion was far more widespread among these cultures however.
Other instances of the valknut in Viking ornaments are Lärbro stone, River Nene ring and a bedpost found on the Oseberg ship. However, knot of the slain is not the only possible interpretation of the valknut. It is also called Hrungnir’s heart. This name is based on a description found in the Prose Edda:
It also stands for extreme bravery and a willingness to die in a fight or sacrifice one's life to protect family and folk. The vast majority of Odinists and Ásatrúars discourage anyone who is not of their faith from getting a valknut tattoed on their skin – it's a serious statement of the Odinic path someone has chosen in life, and it can often be a lonely and harsh one. Not every runenmeister (let alone every Ásatrúar) ends up making this level of commitment to Odin's path. And those who do, are well aware that offering yourself as potential Einherjar material means accepting the risk that you may well die a very painful death.
The following is commentary of some Ásatrú watchers who decided to add their two cents to the issue:
This is a very serious symbol, not for cowards, the hall of Odin is not for cowards, if you're willing to "Do right and fear no one" then this symbol is for you, if not, then don't wear it, or someone might come by and take it from you, along with whatever it's attached to.
Those who wear the Valknut have pledged an oath to Odin himself [whether they realise it or not] to live their lives as a warrior, and woe to him that breaks that oath!!
'”That I advise you secondly, that you do not swear an oath unless it is truly kept; terrible fate-bonds attach the oath-breaker; wretched is the pledge-criminal.'” - Sigrdrifumal 23.
However, whereas a valknut pendant can be taken off, tattoos are permanent and basically like signing in blood. You are marking your own flesh permanently with the holy Knot of the Fallen, of the warriors oathed to Allfather himself, and literally writing Odinism into your blood, beneath the skin! The valknut is not a universal obligation, even most Heathens do not go as far as tattooing it, and those who do are firmly convinced that Odin has called to them. It is a marker that you are willing to follow the often difficult and dangerous Odinic path. Make sure you know for certain, if you are or if you are not. And if not, then don't get it done.