The blood forge that is used to corrupt the pristine vulpite can be found in the loot given from the defence missions. You use this alongside red element shards which can be broken down from red element to corrupt the pristine vulpite and give to your veilwyn to evolve it.

To tame it you need the vulpite from the Solwyn then put that in a blood forge and infuse it with red element shards then put that in its inventory which will give it the adventure status effect kill things until that reaches 100% then feed the corrupted Vulpite to it then you’ll have your Malwyn