Does anyone have an idea what is happening to this...

Yellow between the veins / veins stay green / at the base of the plant = nitrogen (N) or magnesium (Mg) deficiency The large dark areas + the random yellow dots worry me more. Could spider mites. -> Check for small brown-ish dots under the leaves, like @LaBOSSANOVA said. Under magnifying glass, you should see microspiders. I recently had an invasion over my basil with similar symptoms. Neem oil killed my basil in a few days by the way. One of your pictures shows a leaf with a much larger dark spot: brown necrotic tissue. Have you under-watered by any chance? Upper level, some of the the veins are affected also, as it shows with some scorched tissue - could be a mild calcium deficiency.