Water deeply and infrequently. If the water ponds up, then fill with sand or ground topsoil (no clods) to bring the area up to level. Normal grasses won't grow in a swamp.

Mulch mow at the mower's highest setting all the time.

Fertilize now with an organic fertilizer like Milorganite, or soybean meal or alfalfa pellets (all at 15 pounds per 1,000 square feet). You might not be able to get Milo in Canada.

If you refilled the hole with wood chips mixed with soil, you might have a problem for 10 years or more. When wood decomposes the decomposing fungus uses lots nitrogen. Normally it gets the N from the air, so if you bury the wood, the fungus will "rob" the N from all other sources including any fertilizers you put in. The only way to fix that situation is to dig out all that wood chip soil and get rid of it or use it on the surface to mulch flowers. Refill the hole with regular topsoil or sand.

Thank you so much! That is great advice on the mulch I never ever thought of it sucking the nutrients.

I have put some more thought into this, I think digging it out as you suggested is the only option - everyone told me leave it a little high because the soil will settle and sink, so I did, but the little mound I created actually causes the sod to get scalped to like an inch or two because of the angles (even with my mower on highest setting) and this obviously put a ton of stress on the sod. That in combination with me thinking it was dying and over watered the sod plus the bits of dying tree below it has probably doomed the situation. Once sod is more available I will dig it up clean it out, add new soil and try again.

I was worried it was disease/pest related, I'm very relieved it's not.

Thank you again!

The concept is there but the details are just a little sketchy :-) Wood chips or sawdust mixed in with soil don't "rob" nitrogen as they decompose - they just tie it up or make it immobile. The products themselves actually contain N in decent quantities but the activities of the soil organisms that digest and decompose the wood use up all that N rapidly. And they also pull any extra they may need out of the soil. Once that decomposition process is completed, all the N is returned and made available. And it doesn't take 10 years, you do NOT need to remove all the soil nor will the process consume all the N in any ferts!! The smaller the wood particle, the faster the decomposition takes place and the quicker the N is released from its immobility. Generally, the recommendation of an application of a high N fertilizer for a couple of seasons will very satisfactorily offset the temporary N immobility.