A SEA battle involving a Viking named Onund Treeleg in which stones were hurled from cliffs to crack the skulls of foes below sounds like the stuff of epic historical fiction.

Yet, according to a 14th-century Icelandic saga, this encounter took place off Scottish shores. Is it possible to pinpoint exactly where?

Grettir’s Saga is the story of how the eponymous Norseman’s relentlessly violent and individualistic deeds put him at odds with society, ultimately leading to his death but also to a kind of glory.

The saga’s opening chapters set the scene, telling how Harald Fairhair won the Norwegian crown through battle and how those who opposed him were forced to flee Scandinavia for Orkney, Shetland, the Hebrides and beyond.

One of these westward outcasts was a man of tremendous strength and cunning named Onund. Grettir’s Saga tells how Onund made his base on Barra from AD 871, using it as a launching pad for raids throughout the Hebrides and Irish Sea.

The Saga is quite casual about this, saying how Onund met “many of his friends” and spent three summers raiding around Ireland and Scotland in the same nonchalant tone you might reserve for describing a lads’ weekend. Onund sailed to Norway to fight and lose to Harald Fairhair at the sea battle of Hafrsfjord, likely in 874, and returned to Barra to lick his wounds.

It was these very wounds which gave Onund his unforgettable byname, Treeleg. During the fight, Onund was boarding Harald’s ship when his leg was cut off below the knee. Rather than retiring, Onund adopted a wooden prosthetic and got right back into the Viking lifestyle.

This is where our mystery battle enters the story. Onund Treeleg and his ally Thrand heard that a rival set of Vikings, led by Vigbjod and Vestmar, were sailing for the Isle of Bute. Fights between Viking factions were par for the course in the late ninth century.

Onund and Thrand set out for Bute with five ships while Vigbjod and Vestmar had either eight or 13 depending on the version of the Saga you go by. Either way, clever tactics would be needed if they were to stand any chance.

Luckily, Onund was “the wisest of men” and had just such a plan. Arriving somewhere off the coast of Bute, he arranged his five ships in “a deep narrow channel between two cliffs”, with “plenty of deep water behind them”. The channel had a small island on one side, creating a choke point which negated Vigbjod and Vestmar’s numerical superiority.

Some of Onund’s men went ashore and gathered stones on high ground overlooking the channel, ready to hurl them down as the ships locked together in combat.

Seeing who they were up against, Vigbjod and Vestmar’s men hurled insults at Onund: “Treefoot, Treefoot, foot of tree, Trolls take thee and thy company!”

Certain of victory against an outnumbered, one-legged opponent, they sailed straight into Onund’s fleet. After a bit of bloodying, Onund’s ships began drawing back under the cliffs, appearing to be fleeing the fight.

As the pursuers reached the cliffs, stones began raining down on them, leaving many “so hurt they might not bear weapon”. Vigbjod and Vestmar’s ships were huddled together, making their crews easy targets. Onund himself surged forward and made straight for Vigbjod.

Vigbjod slashed Onund’s shield, cleaving a great chunk from it but his second swing got stuck in Onund’s prosthetic. There must have been a brief realisation of instant karma before Onund’s sword hacked Vigbjod’s arm clean off at the shoulder. The battle was won, and Thrand and Onund returned to Barra laden with plunder.

Let’s zoom out a little. What were two Viking factions doing slugging it out in the waters around Bute in the late ninth century? At that time, the Clyde was a multicultural highway for peoples from various declining and emerging kingdoms.

There was not yet a “Scotland” or an “England”, as the Gaelic kingdom of Alba – the precursor to Scotland – was first mentioned around 900 AD, two decades or more after this battle took place.

Britons, Northumbrians, Gaels, Norse and others contested these waters. Bute remained a point of conflict between Scotland and Norway well into the 13th century, with a Norse assault on Rothesay Castle in 1230 and the Battle of Largs settling the matter in favour of the Scots in 1263.

Nearer to Onund’s time, in 870, a Norse fleet including the legendary Viking Ivar the Boneless sailed up the Clyde and captured the mighty capital of the Strathclyde Britons, Dumbarton Rock. Vikings had been raiding the shores of the Clyde long before this. In the early 790s they sacked the monastery of St Blane’s in the south of Bute.

KEEN history sleuths will note that this was around the same time, and possibly even before, the “official” beginning of the Viking Age according to English chroniclers, conventionally said to have begun with the famous raid on Lindisfarne in 793.

Given all this, it’s no surprise to learn that two Viking fleets chose these waters as their battleground at some point during the late 870s.

The key question remains – where was this battle fought? While no smoking gun (or, perhaps, bloodied axe-head) has yet been found, geography narrows down the options.

Off the very northern tip of Bute where the Kyles of Bute and Loch Ruel form a wishbone shape lie several small islands. There are the three Burnt Islands, one of which was topped by an Iron Age fort whose stone walls were fused by fire in a process known as “vitrification”. West of the Burnt Islands at the mouth of Loch Ruel is Eilean Dubh, the “Black Island”.

Just as Grettir’s Saga describes, at several points these islands create narrow channels backed by deep water. There are no other similar locations off Bute’s shores.

The disqualifying factor for the Burnt Islands is that they are very low-lying, with nothing anyone could describe as “cliffs” without considerable creative licence – which the Norse sagas certainly indulged in. Another island contender, Eilean Dearg, has neither sufficiently high ground nor enough room to conceal stone-hurling warriors to be in the running.

That leaves one option: the waters around Caladh Harbour and Eilean Dubh. Here, all our clues seem to fit into place. Stretches of raised rocky shorelines do indeed provide a heightened lookout over the narrow channel, with Loch Ruel and the Kyles affording plenty of deep water just as Grettir’s Saga describes.

Allowing for a little exaggeration about the cliffs, and taking each of the clues about the battle’s location in turn, this is by far the best fit.

While nothing may be left from this 12th century-old sea battle to decide things once and for all, if I had to search for it somewhere, this is where I’d start.