Ulcers are sores on the body which have been caused by a bacterial infection. You need to make very sure your water quality is immaculate. Check your pH and Ammonia levels as well as your Nitrite readings. Then, salt the pond at the rate fo 5% to 7% (1/2 ounce to 3/4 ounce per gallon). 

Make arrangements to diagnose or treat (or both) the Flukes which almost certainly helped this problem come to a head.

If you'll make sure water quality is good and treat the fish using a good topical dressing such as Koi Doc and if possible an anti-biological injection, hopefully you have a good chance for success.