Doom, played very slowly, some quiet soft bits, some very loud bits... but most importantly for you it features only clean vocals (almost bordering on Gregorian Chants at times...)
I watched both 'Lost Kingdoms' and 'Vikings' series last year. This tune would fit either of them during contemplative sections very well, esp. with the recital of Cuthbert that starts during the middle. They should look to bands such as this for some atmosphere.
If it's Anathema, then it has to be this, esp. the latter part. Hope you already know this one.