You were not offered to buy the land, yes? You are willing to buy but the owner/heir is not willing to sell, they actually retained a law office to initiate ejectment against you.
Do you need proof of ownership from the heir? When you yourself said that the claimed owner is the heir of the deceased owner of the land, yes?
Are you in a highly urbanized area? (for rent control)
What do you do?
Do you want to stay on the land? Offer to buy it at fair market value. It's the price at which the seller is willing to sell, and you are willing to buy. If you cannot come to an agreement, then what the owner/heir may do is have you ejected from the lot. They may not have a lease agreement but there may be papers or affidavits or receipts that will show that you're only renting the lot.
The problem is that the owners/heirs want to use the lot but the tenant for 32years is refusing to leave. For example, if you're renting an apartment and the owners want to use the apartment, you are really given a number of days to vacate the place.
You may go to court (as a last resort, if you really want to fight to stay there and you think you have a claim) but the process is lengthy and expensive, and there is no guarantee of a win.