It's a black shirt with an image taken from The Bayeaux Tapestry (11 century) depicting a detail of the legend ubi unus clericus et Ælfgyva, which is mostly fogged over by time. Nobody knows what and why. But here it appears as a depiction of the feeling of reaching through the veil into another world while still standing on the ground of this here and now. Touching the spirit realm right on the face. What all these songs are trying to do.
It also says Mount Eerie really big.
The garment was made totally in the USA out of 100% cotton by Bayside and masterfully printed in Anacortes by Paul of Granular.
(Image is pretty big on there. The shirt in the photo is a "large".)