Edit: This symbiote is a special type that makes its host to produce webs, rather than using itself to make them. It then stores the webbing in its own abdomen for later use.

There are many types of symbiotes in the Marvel Universe. Many types and kinds. However, one of the rarest is magic symbiote. Symbiotes attracted to magic creatures or beings in order to feed on and amplify their magic. They don't really have a preference for normal flesh and blood, and often stay invisible, so not many know about them.

Some types of magic are unappealing/ unappetizing to them, such as Dr. Strange's magic and the Eye of Agamotto. Others however, are very appealing. The strange thing about magic symbiotes is that they don't behave like normal symbiotes. They can teleport across dimensions to catch their targets... and Runeterra happens to be where one of them went. The name of this symbiote is Arach, a special symbiote that is adaptive to magic, and can even use, produce and consume it.
This powerful symbiote has found its way, forcibly, into the Freljord, and straight onto Willump's unsuspecting body via trap.

Willump stumbled into a sub-dimensional pit, where the symbiote closed in around him for breakfast, lunch and dinner. It eagerly took over his entire body, inside and out before taking almost full control over the Yeti. Though others were tempting, Willump was the most appealing because he was already like a spider, having 6 limbs. Being the only non-savage Yeti also made his magic signature stand out like a beacon to the symbiote, who was now eagerly taking him over, coveting and desiring that magic for itself! Normal symbiotes would have frozen in the cold, but Ararch was immune to such conditions.

Willump almost yelped as he hit the nearly invisible web; only lighting up when it resonated with his magic! He tried to resist, but the symbiote was very persuasive, and very controlling! In just seconds, the symbiote had adapted to his magic and merged completely; not only on the outside either! The Yeti's chubby body was being pumped and inflated with the Symbiotes bulk!
What started out as a thin, slime layer of slimy mass the symbiote began to glow, bulking up and hardening, molding and contorting to fit Willump's body.
Willump, use to Runeterra magic, couldn't properly resist the mind control he was being subjected to... enveloped in a half-aware mindset as the symbiote filled the other half. He had no idea what was going on. All he heard was a voice in his head.
"We will be one... and we will not fall!" the symbiote repeated over and over, louder and louder as its voice reverbed, making the Yeti dizzy as his mind lost control over his body.
When it was done, the symbiote began to test its host body, wiggling his toes, shooting webs and bouncing on it web, before choosing to roam the Freljord to mark his territory. He was not going to let the Yeti go for another host, and probably not at all, as an abdomen/spider tail over Willump's original one. Even though he has a spinneret now, he can shoot magic webs from anywhere.

With his body and mind fused to the Symbiote, the two began to travel the Freljord, being much stronger than before. Nunu is in for quite a time when he meets the new Willump.