I am interested in learning more about why/if the Varangian Guard was an effective fighting force, and if so, whether their military tactics were unique in the Byzantine Empire. Most of the sources I've found discuss how opponents of the Varangian Guard were fascinated by their appearance, and fearful of their ferocity. However, there is little to no discussion of their tactics as a unit, and whether the Norse recruits brought over military 'know-how' from Scandinavia.
For example, I found the following on Wikipedia:
The Varangian Guard was only used in battle during critical moments, or where the battle was most fierce. Contemporary Byzantine chroniclers note with a mix of terror and fascination that the "Scandinavians were frightening both in appearance and in equipment, they attacked with reckless rage and neither cared about losing blood nor their wounds". The description probably refers to berserkers, since this state of trance is said to have given them superhuman strength and no sense of pain from their wounds.
I am suspicious as to whether this is correct. My understanding is that modern scholarship on berserkers suggests the view of these warriors as intoxicated, bloodthirsty fighters is not entirely accurate, and the reality is more complex. Further, there is no discussion here regarding the techniques used in battle (such as shield walls, weaponry, etc).
We don't know.
We actually know very very little about the Varangian guard as an institution, as most of what survives and is available in the wider world of English history writing comes from a smattering of scant references in Greek sources (as in we are talking about a page and a half from Anna Comnena's massive chronicle, a smattering of Norse saga stories that were....not exactly intimately familiar with Byzantine military/political culture, and limited archaeological evidence)
Trying to divine the tactics that they employed from the small source material is pointless. There is nothing to indicate that they fought in shield walls or employed berserkers (indeed the actual existence of berserkers as a force on the battlefield and not just a literary flourish of Icelandic writers is not exactly ironclad). In all likelihood the varangians fought as a part of the infantry in the Byzantine army, Anna Comnena backs this up, and took positions on the flanks of the Byzantine army at times, or could be deployed more flexibly, and later in Byzantine history, their prominence and proximity to the emperor placed them in prominent roles, especially under the Commeni emperors, however this are quite simply not enough surviving accounts, or details within them, to describe the specific tactics, arms, or equipment of the varangians.
There are a plethora of Byzantine military treatises that detail what the Byzantine infantry should be used for, how they should be armed, and what their purpose on the battlefield was to accomplish, but they make no real mention of any special role that the varangians fulfilled. I leave it to a Byzantinist to describe the role of infantry in Byzantine armies specifically, but I can say that the varangian guard remains largely obscure in how it was deployed on the battlefield and in the finer points of its equipment, structure, organization, etc...
Should anyone come across this thread in the future, I found an excellent (although expensive) book on the Varangian Guard: The Varangians: In God’s Holy Fire (New Approaches to Byzantine History and Culture).